The Portuguese boss has played Luke Shaw at left-back in each of his games in the Premier League, but has opted to give the Englishman some rest as the club head into a busy month of fixtures.
Rojo had not featured in any of Man United’s league games this season, but did manage to make a substitute showing in the Community Shield win over Leicester City.
Speaking in his press conference, Mourinho said it was a simple decision to make.
“My option in the last four games in the Premier League was Luke, Luke, Luke, Luke,” he said.
“I think it is time for Marcos to play so it was a simple decision.
“Marcos is a very good player who is working really well and hard.”
Rojo joined Man United two years ago under Louis van Gaal, but has struggled to cement his place in the first-team.
Despite being hampered by injuries during his time at the club, he has made 53 appearances and is set to gain some more minutes on Thursday night.
He hasn’t been able to win over fans and was linked with a move away from United this summer, but if he can show some consistency with his defending, it may be enough to impress the Portuguese boss.