The summer transfer window has been open for business since June 16 and clubs in Europe's major leagues can do deals until 2 p.m. ET (7 p.m. BST) on Sept. 1, but some leading players risk becoming high-profile "untransferables" in the weeks ahead.

From Marcus Rashford to Darwin Nunez and Raheem Sterling to Jack Grealish, some of football's biggest names face an anxious summer waiting for a new team to offer them a new challenge after falling out of favor at their parent club. It is a problem that has become particular to Premier League players because only a select group of clubs from outside football's biggest league can afford to sign them or pay them -- and those teams, including Real Madrid, Paris Saint-Germain, Bayern Munich and Barcelona have other targets.

So, with clubs looking to strengthen their squads before those not participating in the FIFA Club World Cup return to preseason training in early-July, which players are struggling to find a new club, and what is most likely to happen to them?

Marcus Rashford

Manchester United have plenty of problems to solve this summer, but none is bigger than the challenge of finding a club to take Rashford and his £325,000-a-week salary away from Old Trafford.

After failing to measure up to the demands of coach Ruben Amorim, the 27-year-old has not kicked a ball for United since the Europa League tie against Viktoria Plzen last December, and he managed just four goals in 17 appearances for Aston Villa during a six-month loan spell in the second half of last season. Rashford is keen to move to LaLiga champions Barcelona, but their interest in Nico Williams has reduced the chances of the England forward landing at Camp Nou this summer.

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With three years to run on Rashford's contract, United would need to bank a fee of at least £45 million just to avoid recording an accounting loss on the player -- a crucial issue in terms of complying with Profit and Sustainability Regulations (PSR) -- and that looks highly unlikely. Another loan move could be the most realistic outcome for Rashford and United, but the worst-case scenario is the player returning for training next month and Amorim having to deal with the distraction that would create.

Darwin Núñez

The Liverpool forward is available for transfer this summer after spending the majority of last season as a bit-part player for Arne Slot's Premier League champions.

Despite scoring just 40 goals in 143 appearances since a club record €75 million transfer from Benfica in 2022, Liverpool value the Uruguay international at €50 million and that valuation has so far priced out interested parties. Napoli have a confirmed interest in the 26-year-old, but the Serie A champions must resolve the future of Victor Osimhen, who spent last season on loan at Galatasaray, before pressing ahead with a deal for Nunez.

Having already spent in excess of £150 million this summer on Bayer Leverkusen pair Florian Wirtz and Jeremie Frimpong, with a £40 million move also planned for Bournemouth's Milos Kerkez, Liverpool now need to raise funds to balance the books.

So Nunez could be forced to wait for his valuation to drop before finding a move away from Anfield.

Jack Grealish

The Manchester City winger has been left out of the club's squad for the Club World Cup with manager Pep Guardiola paving the way for a move away from the Etihad by saying that the 29-year-old needs to play.

Grealish has struggled for form and fitness ever since helping City achieve a Premier League-Champions League-FA Cup treble in 2023, and his absence from the Club World Cup echoes his failure to be named in England's Euro 2024 squad last summer.

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Although City are prepared to offload the former Aston Villa player, Grealish won't come cheap with two years still to run on his £15 million-a-year contract. And having been a £100 million signing -- the biggest in Premier League history at the time -- when he arrived from Villa in 2021, City will also expect a significant fee for any permanent transfer.

Newcastle and Everton have shown an interest, but as it stands, City have yet to receive a formal inquiry.

Antony

Antony has earned the unenviable distinction of becoming one of the biggest transfer flops of the Premier League era since completing an £81 million move from Ajax to Manchester United in August 2022, and the 25-year-old's failure to perform at Old Trafford is why the club is struggling to offload him permanently. The Brazil international enjoyed a successful six-month loan at Real Betis last season, helping the LaLiga team to the UEFA Conference League final, but with United needing to bank a fee of at least £32m to avoid recording an accounting loss on Antony, Betis cannot afford a permanent deal.

Ultimately, his fee is the major stumbling block to any transfer. His £70,000-a-week wages make him relatively inexpensive as a loan option, but United want to cut their losses by getting him off their books.

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Bayer Leverkusen, now coached by former United manager Erik ten Hag, and Betis are in the market for a loan deal, but at this early stage of the window, United have no takers for a permanent transfer.

Kyle Walker

Manchester City fullback Kyle Walker has no future at the Etihad, but a January loan move to AC Milan failed to turn into a permanent deal for the 35-year-old. With one year still to run on his City contract, this summer offers the club their last chance to generate a fee for the England defender and there is some interest from Turkey, with Fenerbahce considering a move.

Walker's failure to impress in Serie A has diminished his chances of getting a move to another major European league and Sheffield United's failure to win promotion back to the Premier League ended the prospect of a return to his boyhood club.

City have still to find the solution to their right back problem, but it is highly unlikely Guardiola will do that by giving Walker a reprieve.

Raheem Sterling

At 30 years old, Sterling should have a raft of options after falling out of favor at Chelsea, but the reality is that the former England winger has become a forgotten man who nobody wants. A return of just one goal in 28 appearances -- mostly as a substitute -- during a seasonlong loan at Arsenal ensured no prospect of a permanent deal at the Emirates.

But with two years to run on a £300,000-a-week contract at Chelsea, Sterling must either accept a huge pay cut to leave Stamford Bridge or face up to two more years of loan spells.

Sterling has made it clear that he is not interested in a move to Saudi Arabia, while a switch to MLS depends on Chelsea waiving a transfer fee and the player accepting a substantial drop in wages.

Gab Marcotti reacts to Jadon Sancho's decision to return to Man United after his loan at Chelsea.

Jadon Sancho

Chelsea had been prepared to hand Jadon Sancho a permanent contract after triggering their obligation to sign the winger following his seasonlong loan from Manchester United, but the player's reluctance to reduce his £300,000-a-week Old Trafford salary scuppered the deal. So Chelsea paid United a pre-agreed £5 million penalty clause for failing to fulfill their obligation, and the former England international is once again facing a summer of uncertainty at his parent club.

Sancho has a year to run on his United contract -- the club has an option to extend it by a further year, but don't expect that to happen -- and has no future under Amorim, so United have a Sancho problem to deal with once again. Fenerbahce have been linked with the 25-year-old, while Juventus and former club Borussia Dortmund retain an interest, but United's best hope is likely a loan deal that covers at least half of Sancho's wages.

Christopher Nkunku

France international Christopher Nkunku has managed just 17 goals in 58 games for Chelsea since completing a £52 million move from RB Leipzig in 2023. The Blues had been keen to offload the 27-year-old in January, but a deal did not materialize and Nkunku is once again available for transfer.

Injuries have sidelined Nkunku for over 50 games during his two years at Stamford Bridge and his patchy fitness record will impact his chances of a permanent move. But with four years remaining on his Chelsea contract, the onus is on the club to find a club, which won't be easy considering his failure to impress in west London.