Sanchez was ruled out of the clash at Brighton due to a 'little problem', while Marcus Rashford dropped to the bench as the Red Devils look to make it back-to-back Premier League wins.
Martial has been strongly linked with a move away from United this summer, but the Frenchman was named in the starting XI against Brighton on Sunday afternoon.
Mourinho has revealed that Romelu Lukaku's lack of fitness has led to Rashford being held back as a potential second-half substitute, which has allowed Martial the chance to impress from the start.
"We know Anthony well and you can always expect good things from him. He can find that stability that makes the right players, we try to help him and he is working well," Mourinho told Sky Sports News.
"Alexis was going to play but he had a little problem so we had to decide the opp for one attacking player. Jesse [Lingard] is the one that is late on his preparation he had a small injury when he started last week, he was not selected last week.
"Marcus is obviously an important option to play on the left. But we believe that Lukaku can not play 90 minutes. So start with one striker and one on the left so the decision was to protect Marcus for that situation to play later and give Martial the opportunity to play."
Martial scored 11 times and registered nine assists in 45 appearances for United last season.