Marcus Rashford has praised Mourinho for his orders, which he followed when he was brought on, inspiring United to a 2-0 win.
The youngster came on before the hour mark and assisted the opener, with Mata getting the opening goal.
Zlatan Ibrahimovic added a late second for United against a West Ham side reduced to 10 men after just 15 minutes following Mike Dean’s decision to send off Sofiane Feghouli for a challenge on Phil Jones.
Asked what message Mourinho had given him before he went on, Rashford said: “He just told me to stay wide and try to isolate the full-back and we did in the end. I’m just following orders, of course!
“I’m very pleased with the three points.
“We’re in and around the top four. It’s very tight between sixth and second so we have to take it game by game and try to win every game.”