The Spanish midfielder headed straight down the tunnel after picking up a suspected muscular problem during the 1-0 friendly defeat by Bayern Munich on Sunday.
Defender Eric Bailly also picked up a knock and was forced off five minutes into the second half of the pre-season clash.
And Mourinho has admitted that he is yet to learn the extent of the problems suffered by the duo during the game.
Speaking to MUTV, Mourinho said: “For the ones who played lots of minutes, it was hard once more and Ander, we don’t know, but it’s between him and Eric, apart from that, the boys resisted and tomorrow, for them, is a day off.”
Manchester United will take on Leicester City in the Premier League curtain-raiser when they host the Foxes at Old Trafford on Friday night.
Mourinho will be aiming for his side to challenge for the title next season after they ended up in second place and without a trophy last term under the Portuguese coach.
The Red Devils have not won the Premier League title since 2013 and they will be out to try and challenge Manchester City’s dominance at the top of the table next term.
Manchester United had won both the Europa League and League Cup in Mourinho’s first campaign in charge.
MORE: Man United latest news
MORE: Arsenal latest news
MORE: Chelsea latest news
MORE: Liverpool latest news