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

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 outside their own leagues 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 €11m-per-year on offer reportedly short of the €25m 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 in recent years is an issue though and this season's performance, where they look capable again, 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 256 goals in 378 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. The 35-year-old Fernandinho is virtually irreplaceable but also looks set to go.

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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.

Memphis Depay (Lyon)

The Dutch forward has attracted interest from Barcelona and PSG in recent weeks and could be available for as little as €25m. Reaching double figures for goals in the past three seasons, Depay has reinvented himself in Ligue 1 and may be tempted by a step up.

Others: Marseille winger Florian Thauvin is also coming to the end of his contract and could be seen as a viable alternative for a club if they can't land Depay.

Julian Draxler (PSG)

Once tipped to become one of the world's best young players, Draxler has fallen down the pecking order 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 €42m signing from Wolfsburg in January 2017, he was linked with Leeds and Hertha Berlin but couldn't seal a summer move.

Others: PSG could have three spots on the wings open if they let 32-year-old Angel Di Maria depart too, having released Jese already. 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.

Out of contract

These players are already available and just looking for a new club.

Diego Costa, 32: The former Atletico Madrid striker has been linked to a host of top clubs after his contract was terminated early.

Ahmed Musa, 28: Saudi Arabian side Al Nassr let the Nigeria winger go in October and he's been looking for a new club ever since.

Daniel Sturridge, 31: Having had spells at Manchester City, Chelsea and Liverpool, three of the biggest Premier League clubs, Sturridge knows what it takes to succeed. But his career has gone downhill of late.

Alexandre Pato, 31: Once tipped for the very top, the ex-Milan and Chelsea forward has been playing in China and Brazil recently.

Wilfried Bony, 32: Best known for his move to Man City in 2015, the Ivory Coast international returned to Swansea soon after and was released by Saudi Arabian side Al-Ittihad in November.

Ezequiel Garay, 34: A four-year spell at Valencia came to an end earlier this year but Garay still has something to offer and good defenders are hard to find in January.