La Liga's top three teams are inarguably already stacked, but that doesn't absolve them of the responsibility to improve. While some of the biggest squads have already started to make moves in the transfer window, others are biding their time. Here's a look at some of the transfer needs that Spain's top clubs could make before the window closes.

Barcelona

Barca have already been busy in this transfer window, inking Samuel Umtiti from Lyon and Lucas Digne from PSG while sending Marc Bartra and Dani Alves out the door. The one-for-one replacements surely seem like upgrades, but Barcelona could still use another helping hand on defense. In that vein, they could continue their raid of Ligue 1 clubs and plunder someone like Marquinhos from PSG, who can't break through the Parisian club's central pairing of David Luiz and Thiago Silva.

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Another area where the La Liga and Copa del Rey title-holders could turn their attention is at the backup striker position. Bringing in Denis Suarez gives Luis Enrique depth for Lionel Messi and Neymar, but a bona fide backup striker for Luis Suarez wouldn't go unnoticed. Sevilla's Kevin Gameiro could be just the ticket, or possibly Santos star Gabriel Barbosa, aka Gabigol. Neither would be a particularly cheap option, but Barcelona usually figure out a way to get things done.

Real Madrid

What do you get a club that seemingly has everything? Real Madrid narrowly missed out on the double last season, claiming the Champions League title and falling short of Barcelona in the league by one point. There was also their brain-fart aided exit from Copa del Rey, but aside from better clerical work, where can Los Blancos shore things up?

Their back line is only growing older, with Marcelo 28, Pepe 33 and Sergio Ramos 30. Alvaro Arbeloa has called it quits, and 23-year-old Raphael Varane's injury history is dicey at best. With a quiet start to the window so far, would Florentino Perez have the audacity to pull off a Galactico-level purchase? Bayern Munich's David Alaba would check all of the boxes for Real. He's young and he's a natural left back with the versatility to play both center back and central defensive midfield. Would Bayern ever sell the 24-year-old? It's highly unlikely, but stranger things have happened. A more realistic left-side defender could be Valencia's Jose Luis Gaya.

Atletico Madrid

Diego Simeone and Atletico already bolstered their attack with the addition of Nico Gaitan from Benfica, but Los Colchoneros could still use help in that area. First and foremost, fighting off any overtures for Antoine Griezmann remains paramount. Whatever Atleti need to do to keep the 25-year-old -- who scored 22 league goals last season -- happy, they need to do it. Aside from that, finding him another strike partner would be a boon for their success, too.

The Diego Costa rumors don't seem to be going away, but they don't seem to be too likely, either. If the Chelsea forward doesn't push for a move back to Spain's capital, Atletico will need to go looking elsewhere. The issue for Atletico, who navigate the transfer-market waters with a bit more fastidiousness than their mega-money rivals Barcelona and Real Madrid, is that nearly every club could use a striker. Finding a bargain is tough to do. With that being the case, they might have to limit their search to older but still-proven commodities. Plays for Sevilla's Gameiro or AC Milan's Carlos Bacca wouldn't be bad shouts, by any means.

Sevilla

After a disappointing seventh-place finish in La Liga last season, Sevilla find themselves in a state of flux. They've replaced their manager, Unai Emery, with Jorge Sampaoli. Their transfer whiz, simply known as Monchi, has taken a step back to rest. Midfielders Ever Banega and Grzegorz Krychowiak are out the door. It's not all doom and gloom for Sevilla, though.

Their domestic shortcomings aside, Sevilla will be in the Champions League this season, thanks to their remarkable feat of winning a third consecutive Europa League title. As such, they can stand to be spenders on the market during the window. Despite the arrivals of Joaquin Correa, Ganso and Hiroshi Kiyotake, Sevilla could still stand to bolster the center of the pitch with a defensive midfielder -- perhaps Spain international Sergi Darder is an option. Of course, Sampaoli has promised a relentless attack from Sevilla this season, too. In that case, keeping Gameiro suitors at bay might be their biggest priority.

Valencia

As six-time La Liga champions, finishing in 12th domestically is an unusual feeling for Valencia. Then again, considering the Gary Neville experiment was a total farce, categorizing last season as unusual might be an understatement. From a personnel standpoint, where can Valencia stand to improve? A quality right back to help reinforce the defense would be a good start. An injury to Joao Cancelo would be tough to overcome the way the squad is currently constructed.

Another factor will be hanging onto some of their other starters, primarily on defense. Jose Luis Gaya and Shkodran Mustafi's names have both been bandied about thus far in the transfer. At the other end of the pitch, Nani's addition could be an under-the-radar steal, assuming the Portuguese maintains his form from the Euros. Otherwise, breakout striker Paco Alcacer could use cover with a backup in attack.

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