Ralf Rangnick said Juan Mata was a victim of intense competition at Manchester United as his career at the club peters out.
United extended Mata's contract by a year in the summer but the 33-year-old has only played four times this season and his two starts were against West Ham in the League Cup and the final Champions League group stage match against Young Boys - matches where there was mass rotation.
Mata is out of contract in June and almost certain to leave United after eight-and-a-half years while the World Cup and European Championship winner has not played for Spain since 2016.
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"He's also very popular in our group," Rangnick acknowledged. "I think everybody likes Juan a lot and he can still play on this level but it's a question of competition. We have quite a few players for those midfield places and I will have to choose between five and six players for tomorrow's game."
Raphael Varane reiterated his importance to United on his return to the side in the 3-2 Premier League win over Tottenham on Saturday. United have not lost a game that Varane and Harry Maguire have started and finished in together and their heaviest defeats of the campaign against Leicester, Liverpool, Watford and Manchester City came without Varane.
The 28-year-old is set to start again against Atletico Madrid in the Champions League round-of-16 second leg on Tuesday night but Rangnick also lauded Victor Lindelof's cameo against Spurs.
"It's very important but with Victor Lindelof we have also won quite a few games, played well and had quite a few clean sheets," Rangnick explained. "So right now we have top players for the centre of defence, it will not be so easy for me to take that decision for tomorrow.
"Also when Victor came on in the last 10 or 15 minutes he played well. It's good to have Rapha fully fit again, it's also good that we looked after him and he looked after himself and didn't play too early again. Now he seems to be fit on a regular basis and can play for us regularly."
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