United saw off MLS outfit LA Galaxy 5-2 in the first match of their pre-season tour of the US.
Mourinho made wholesale changes at half-time in order to give his squad 45 minutes of action each.
New signings Romelu Lukaku and Victor Lindelof began on the bench as United led 3-0 at the break.
Marcus Rashford led the line and scored the opening two goals before Marouane Fellaini added another.
Lukaku failed to find the net on his debut, having come close twice early in the second half, but the Belgium international was denied on both ocassions.
Meanwhile, attacking team-mates Henrikh Mkhitaryan and Anthony Martial also got on the scoresheet.
Galaxy managed two goals as consolation in the final 10 minutes, both through Giovani dos Santos.
But United were largely comfortable and missed further chances to increase the gap between the two.
£75million summer signing Lukaku will look to get off the mark again on Tuesday (3am) when United take on Real Salt Lake.
That clash will be followed by bigger tests against Manchester City, Real Madrid and Barcelona.
