West Brom's Jay Rodriguez scored the only goal of the game against a lacklustre United.
Manchester United fans were delighted to see their record signing back to his best as he prodded past Ederson twice to level the derby and help delay City's title charge.
After sparking a famous derby comeback against Manchester City days earlier, Paul Pogba endured another miserable afternoon at Old Trafford on Sunday.
"That doesn't make me feel super happy and in my opinion affected for sure some of the people that was too happy".
Despite the injury, Pogba's Premier League campaign has not been as disrupted as Philippe Coutinho's was.
Mourinho has been publicly critical of Pogba on a number of occasions this season. We are a team that in the last three matches [against] Chelsea, Liverpool and Manchester City, has won all of them.
Afterwards, Mourinho said: "I trust in my work, that is for sure".
'The commitment, the desire, and I said, "Can Pogba do that all the time the way he played against City?"
"I think he has to try to keep that level of stability and, from that stability of course will appear the special match with the special performance, like it happened against City". Not just the performance level but also the mental level, the day-by-day, everything must be consistent, since day one until the last day.
"Everything must be consistent until the last day". Everything was slow. Everyone was one more touch, one more trick, one more flick, one more turn.
The second half was not much better but Foster did wonderfully well after 66 minutes to tip a Lukaku effort around the post before the visitors resistance was then unexpectedly rewarded with 17 minutes remaining.
After sacking Alan Pardew at the start of the month, Darren Moore has been handed the West Brom until the end of the season.
"West Brom outworked them; they wanted to go out and hurt them".
"West Brom is a club I care about a lot", he said.