Messi, Alaba, Ozil, Aguero among the top free agents in 2021

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Why Dembele, Sancho & Depay ended up staying put (2:22)

Gab Marcotti and Julien Laurens dive into the most talked about deals that didn't pan out this transfer window. (2:22)

Despite the financial downturn across the globe, clubs in Europe's top five leagues still spent over €4.6 billion in the summer window -- a drop of 43% when compared with 2019. And as the economic reality of COVID-19 hits hard in the next few months, free transfers will become even more important.

Some big-name players will be available on a free transfer in 2021 if they don't put pen to paper soon.

Here are 10 top players who can be picked up for nothing -- and can start to negotiate with other clubs in January -- with the help of Transfermarkt.

Lionel Messi (Barcelona)

Messi's desire to leave the club he's been at since the age of 13 was the story of the summer, but he opted not to take them to court over a clause in his contract saying he could leave for free and not for his €700 million release clause. Manchester City (and former Barca manager Pep Guardiola) missed out on him but surely he'll move in 2021 when his contract actually expires. He'll be 34 but he's still world-class, so his return to Argentine club Newell's Old Boys before he retires is some way off.

Others: Barca's need to overhaul their squad could see them take advantage of some free transfers, too. With Dutch manager Ronald Koeman in place, moves for Liverpool's Georginio Wijnaldum and Lyon's Memphis Depay are on the radar, though they may not happen if Koeman doesn't last the season. The club will also have a decision to make about young midfielder Riqui Puig before his deal expires.

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David Alaba (Bayern Munich)

Alaba has been with Bayern since 2008, but things are starting to turn a bit nasty. Recently, former Bayern president Uli Hoeness accused the agent of Alaba, Pini Zahavi, of being "a money-grabbing piranha" amid negotiations over a new contract for his client -- with the €11 million-per-year on offer reportedly short of the €25 million they want. Alaba is still only 28 and is a world-class defender, so won't be short of options.

Others: Bayern will likely let Jerome Boateng and Javi Martínez, who almost returned to Athletic Bilbao in the summer, go either in January or once their deals expire.

Gianluigi Donnarumma (AC Milan)

It was something of a surprise when the best young goalkeeper in the game wasn't the subject of major attention in the summer window. Donnarumma is only 21 but is a class act and Milan are clearly confident they can tie him down to another deal. Milan's failure to get back into Serie A title contention is an issue though and this season's performance will be key.

Others: His brother Antonio is also nearing the end of his deal, as is midfielder Hakan Calhanoglu, defender Mateo Musacchio and a certain 39-year-old striker named Zlatan Ibrahimovic.

Sergio Aguero (Man City)

The striker has spent almost a decade at City, having joined from Atletico Madrid in 2011, scoring an incredible 254 goals in 270 games. Now nearing the end of his career, Aguero has always been open about where he wants to play next. "I have always said that I want to return to [his former club] Independiente and I will do just that," Aguero told So Foot in 2015. "I will return to Independiente when my contract with City expires." Back then he was talking about moving in 2019, which clearly didn't happen, but now he's 32 the time might be right.

Others: City will also look to sell centre-back Eric Garcia to Barcelona in January to recoup some money, having failed to reach an agreement over the summer, but the 35-year-old Fernandinho is virtually irreplaceable and will surely earn another year.

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Mesut Ozil (Arsenal)

It was a coup for Arsenal to sign Ozil for €44 million from Real Madrid in 2013 but the club surely regrets handing him a £350,000-a-week deal in 2018 as he's become a pariah. Injuries, loss of form and a general lack of desire have seen the 31-year-old midfielder dropped from the first-team squad, while he says he will run down his deal despite the Gunners wanting to move him on.

Others: Arsenal have a host of centre-backs now, so it's hard to see any of Shkodran Mustafi, Sokratis or David Luiz getting a new deal either.

Sergio Ramos (Real Madrid)

Real failed to make a move in the summer window for the first time in 40 years, but when they have players like the 34-year-old Ramos it's easy to see why. The Spain defender joined in 2005 and has become one of the best in the business, so it's hard to see Real not granting him the two-year extension he wants (and deserves.)

Others: Luka Modric almost joined Inter Milan a while back, but the 35-year-old is keen to finish his career in Madrid. However, it's unlikely the club will offer Lucas Vazquez a new deal.

Paul Pogba (Man United)

Pogba is another player whose future has been the subject of much debate. Real Madrid and Juventus have been interested for some time now and would relish the chance to land him for free, though talks have been ongoing over a new deal and sources told ESPN in July that United are relaxed about the Frenchman's long-term future and will exercise the one-year option in his contract if terms have not been agreed.

Others: Less likely to stay on once their deals expire are squad players Sergio Romero, Juan Mata, Jesse Lingard and Marcos Rojo.

Julian Draxler (PSG)

Once tipped to become one of the world's best young players, Draxler has fallen down the pecking order under Thomas Tuchel at PSG.

The Germany winger told Bild: "Last season didn't go well at all. Not on a sporting level and also due to my injuries. Then the season ended due to coronavirus. It was definitely a season to forget for me." A €42 million signing from Wolfsburg in January 2017, he featured in just 11 Ligue 1 matches and, though he was linked with Leeds and Hertha Berlin, couldn't seal a summer move.

Others: PSG could have three spots on the wings open if they let Jese and 32-year-old Angel Di Maria depart too, though he played a much bigger role in the side last season. Defender Juan Bernat needs to earn a new deal once he comes back from injury, but PSG will be more concerned about Kylian Mbappe and Neymar's contracts which end in 2022.

Diego Costa (Atletico Madrid)

Love him or hate him, the combative Costa is a valuable addition to any squad. Atletico's style of play under Diego Simeone seems made for him, while the arrival of Luis Suarez this summer should provide some interesting moments in La Liga -- "One bites and the other kicks," as the 32-year-old said. Atletico have been looking to move Costa on as he's been plagued by injuries since signing from Chelsea in 2017 and probably would have if they had landed Edinson Cavani.

Arkadiusz Milik (Napoli)

This one has turned really ugly as Milik was linked with Juventus, Roma, Tottenham, Fiorentina and Valencia in the summer but turned down the chance to leave and won't sign a new deal. As a result, the Poland striker is being made to train alone until January, says Corriere dello Sport, and doesn't appear to have a future in Naples.

Others: Veteran striker Fernando Llorente surely won't get a new contract at 35, the club will decide on Matteo Politano, while there could also be some movement in defence as Nikola Maksimovic and Elseid Hysaj's deals expire too.