Following the debacle defeat against Tottenham on Wednesday, Mournho made four changes to his side, with the biggest surprise being the incluson of McTominay in midfield at the expense of £89m man Paul Pogba.
Pogba will start the game at Old Trafford from the bench for the first time in the Premier League this season, having started in each of his 15 appearances so far.
"I changed a few players but the intention was not to punish anyone", Mourinho said.
"He is very good on the ball and does a great job defensively". I prefer to look at it in the handsome way of a little kid that arrived here at 9 years old with mum for the first training session.
Mourinho has regularly given young homegrown players opportunities and McTominay barely put a foot wrong beyond being disposed at the start of the game.
Virat Kohli's indifference turns to love for Ajinkya Rahane
Du Plessis shared partnerships of 74 with Chris Morris and 56 with Andile Phehlukwayo for the sixth and seventh wickets. When asked if he had suffered any injury while fielding, Du Plessis said, "Just a few battle scars".
Romelu Lukaku volleys in to break the deadlock at Old Trafford.
"We always had control of the game, but at half-time it was 0-0", Mourinho told BBC Sport.
The visitors used every tool in the box to try to prevent the ball from finding the net and were extremely fortunate not to concede a penalty when McTominay was flattened by Terence Kongolo's wild challenge inside the box. We don't pick the ball in the air - second mistake.
The hosts' first chance of the contest arrived after only eight minutes as Jesse Lingard burst through the middle of the park before drilling a powerful low strike towards goal which Lossl got down well to save.
"It was hard at half time because it was 0-0", Mourinho said.
Bottom club West Bromwich Albion lost 3-2 at home to Southampton on an emotional day at the Hawthorns, where there was a minute's applause for the home side's former striker Cyrille Regis, who died aged 59 last month.