While that could lead to his exit as he seeks starts to make his World Cup case, the Blues shouldn't move him to Manchester United.
Ben Smith of the BBC reports no official bid has been made by United for the talented midfielder, but the ailing champions aren't ruling one out. He's emerged as a target as David Moyes tries to revitalize his stagnant roster:
Senior figures at Old Trafford have told the BBC that United have not made a bid for the player. However it is understood that the Spanish international midfielder is a target as Moyes seeks to bring in new faces to freshen up his struggling team.
Former United captain Gary Neville said on Sky Sports he doesn't think Mata would be the greatest fit based on what United has historically done, but what should worry Chelsea is the fact a move for Mata could signal something bigger at Old Trafford:
He's obviously a fantastic football player, there's no doubt about that, but one thing it does do, I suppose, is that David Moyes, Manchester United and Ed Woodward need to sign a big player.
They need to make a noise and if they get him in it will attract potentially other players—and they need three or four players.
What it could be is the start of something to lead to other things, but typically I wouldn't say he fits.
Chelsea should look at two situations from this season as a warning before even considering any bid for Mata from United or any other top Premier League club.
The first is the impact Mesut Ozil had on Arsenal. Gunners manager Arsene Wenger was under a lot of pressure to make a move to bolster his squad, but instead of wasting transfer funds on second-rate options, he waited for the right opportunity and pounced for Ozil.
It was a terrific coup. The German midfielder instantly improved the Arsenal attack, helping transform the squad from fringe contenders into legitimate title hopefuls as they sit atop the Premier League table through 22 matches.
That's not to say Mata is on the same level as Ozil. While he did tally 20 goals and 35 assists across all competitions for the Blues last season, he isn't as complete of a midfielder as the Arsenal star. But he is a very strong attacking option with plenty of creativity, which United have lacked.
What Chelsea should see is that one player can change the entire outlook for a club. Ozil represented exactly the type of player Arsenal needed and his arrival was a huge boost.
It's no secret the Toffees' biggest problem in recent seasons was the lack of a reliable striker to headline the attack. They were solid everywhere else, but without someone atop the formation to finish the chances, they remained a non-threat.
Suddenly, with Lukaku building off his success with West Brom last season, Everton have become the toughest team to beat in the league with just 2 losses in 22 matches. They are also within striking distance on the top three, in large part because the striker has nine goals in 18 EPL appearances.
Chelsea didn't have a clear path to playing time for the Belgian with Samuel Eto'o and Fernando Torres in the mix. Yet, by sending him to Everton, they created an extra contender for a Champions League berth that probably wouldn't have otherwise existed.
Both of those cases, Ozil providing a noticeable boost to Arsenal and Chelsea watching Lukaku help Everton remain in contention, serve as cautionary tales if Manchester United do come calling about Mata during the winter window.
As Ian Darke of ESPN points out, it could be a sign Chelsea don't view United as a threat this season, but that doesn't eliminate the risk.
It's understandable the Spanish midfielder might want to join a club where he'd have a chance to make a bigger impact. The competition for World Cup spots for the defending champions is fierce and spending a lot of time on the sidelines isn't going to do him any good.
At the same time, Chelsea don't want to provide the spark that could help Manchester United turn things around after a sluggish start to the Moyes era. If Mata is going to move on, they should do everything in their power to make sure it's not to a direct rival.
Otherwise, it's a transfer that could come back to haunt them in the end, whether it be this season or beyond.