Martial, 20, impressed in his first United season after his move from Monaco for an initial fee of 36 million pounds ($44.9 million, 42.4 million euros), scoring 17 goals to finish as the club's top scorer.
But he has scored only twice this season, has been in and out of the starting XI and was left out of the match-day squad entirely for Thursday's 4-0 Europa League win over Feyenoord.
"First of all, the teams are very different," Mourinho said in reference to the changes he has enacted since succeeding Louis van Gaal as manager.
"The way the team played last season -- I'm not saying better or worse, I'm saying just different. So please write properly my words. Different teams and probably more adapted to Anthony (last season).
"Probably he was feeling more comfortable playing that way, at that intensity, at that rhythm, with the ball possession and ball circulation. So probably he needs also some time.
"But in our squad, especially for these positions, we have (Henrikh) Mkhitaryan, (Jesse) Lingard, (Memphis) Depay, (Juan) Mata, (Marcus) Rashford, Martial.
"It's probably the positions where we have more options. You have one opportunity, you have two, you have three -- if you don't bite, somebody comes and takes the meat."
Mkhitaryan made his first start since September 10 against Feyenoord, playing in Martial's usual left-sided role, and was named man of the match after a display brimming with attacking endeavour.
United's win left them one point from the Europa League knockout phase and they now turn their attentions back to the Premier League, with West Ham United due to visit Old Trafford on Sunday.
United are sixth in the table, a sizeable nine points below leaders Chelsea, Mourinho's former club.
But Mourinho cited the example of Chelsea's 2014-15 title-winning season -- which he feels has been overlooked amid criticism of his mixed start at United -- as a reason for optimism.
"We go match after match," Mourinho told reporters after Thursday's win over Feyenoord.
"When I won the last title, 18 years ago -- sorry, 18 months ago -- I had 10 points' advantage and then in one month I had the same points as Man City. We lost 10 points in one month.
"Then we recovered and won the title 18 years ago -- sorry, 18 months ago. So you can recover points. Other teams can lose points.
"During the season there are moments where everything goes against you. There are moments where everything goes in your favour. So we know the game is not over."