The Belgian international arrived from Everton earlier this month and made his United bow on Saturday evening in a 5-2 win over LA Galaxy as the Red Devils kicked off their pre-season in style.
But it was Rashford, rather than the 24-year-old, who caught the eye the most grabbing two goals as his fledgling career continues to go from strength to strength.
Rashford, who is also an England regular, continues to improve apace and, despite his goal tally dipping last term, United manager Mourinho is understandably excited by his potential.
"It's just the natural evolution of a kid," he said. "Accumulation of minutes on the pitch and you change your body when you're a young kid so that's absolutely normal. He's already taller than when I arrived 13 months ago, he's three centimetres taller, and obviously he put on some muscle, but without any kind of specific work because his speed is the most important quality so we don't want to lose that.
"Today he was in a different team than (Romelu) Lukaku. In this pre-season we will also try both in the same team which I think can also work."
Playing big-money signing Lukaku alongside Rashford is a tantalising proposition, especially given the fact Mourinho expects the 19-year-old to find the net with more regularity this term.
"Last season in this period everybody was saying that with me it would be difficult for him to play," the United boss said. "In the end, I think he was one of the top three with more appearances for the team so he played a lot, he developed a lot. He played in different positions, which is good for his evolution.
"He played the important Europa League matches, which is like a pre-Champions League, and he's learning a lot. He is a great professional with a great attitude and last season for too long he was in a period of not scoring goals and missing a lot of chances, which was not a drama because sometimes that's part of the evolution. We knew that this season he is going to score more goals than last season."