Special One believes opening-day clash proved Red Devils have learned their lessons from last season.
Old Trafford fans grew frustrated with a number of stalemates in the previous campaign – an issue the boss signed Romelu Lukaku to fix.
Mourinho, 54, said: “The game was open at 2-0 and they had a chance. “I don’t think the game was 4-0 of course you score the third and you breathe and you relax, you play with a lot of confidence – 10 more minutes and we could have scored more. “I think we didn’t start especially well. I think we were nervous, not so confident but then we had a quality performance good personality and we looked confident.
“It’s nice for Morata to score, nice for Wayne Rooney to score, nice for Lukaku to score, Lacazette and some others that I don’t know. “We lost two points against West Ham here last season. We had matches where we were dominant but couldn’t score enough goals. Today we did and got three important points. “The reality of it is in the last part of the game instead of being boring and controlling the game we scored the third goal and everything is open to enjoy, yes I’m happy.”