The Red Devils were handed a boost in the first half after West Ham midfielder Sofiane Feghouli was sent off in the 15th minute but the visitors couldn’t make their numerical advantage count.
Mourinho brought on Mata at half-time before he turned to Rashford in the 60th minute and the England international teed up the £37.1m midfielder to break the deadlock with 27 minutes remaining.
Zlatan Ibrahimovic continued his fine goal-scoring run as the 35-year-old centre-forward added the away side’s second to seal three points for the 20-time English champions on their first visit to the London Stadium.
Speaking to MUTV after Manchester United’s 2-0 win against the Hammers, Mourinho was full of praise for Mata and Rashford after their impact from the bench.
“It was a difficult match but I had ammunition on the bench,” Mourinho told MUTV.
“I had Mata and Rashford, who were crucial in both matches when coming off the bench. In these fixtures it is very important to have people on the bench, capable of winning the game.
“I had a feeling to start Mata and Rashford today, but I think it is better for them to come from the bench when everyone is dying and the decision was proven right.”
Manchester United finished in fifth place last term.
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