The French forward suffered the problem in training on Friday and sat out the Red Devils’ 1-0 win over Watford in their season finale at Old Trafford on Sunday.
The injury inevitably casts doubt over whether Martial will be fit to face Chelsea FC in the FA Cup final at Wembley next Saturday afternoon.
And Mourinho delivered the latest update on Martial’s knee injury after Manchester United’s 1-0 win over Watford this weekend.
Speaking at his post-match news conference on Sunday, Mourinho said: “Anthony, I have to wait a bit for more. It’s his knee,.
“He was injured in the Friday training session and yesterday [Saturday] he couldn’t train.
“He had treatment in the morning at the training ground and he stayed away.”
Manchester United finished in second place in the Premier League this season in what was their highest finish since Sir Alex Ferguson’s retirement back in 2013.
The Red Devils will now look to end the campaign on a high by beating Chelsea FC in the FA Cup final at Wembley on Saturday as they bid to win their third major trophy under Mourinho.
The Portuguese coach, who took over at Old Trafford in the summer of 2016, led Manchester United to the Europa League and League Cup titles in his first campaign last season.
MORE: Man United latest news
MORE: Arsenal latest news
MORE: Chelsea latest news
MORE: Liverpool latest news