A 73rd-minute header from Jay Rodriguez gave Albion, who appear doomed to relegation, their first away win in 16 outings and just their fourth victory of the season.
City displayed a strong mentality to bounce back from three consecutive defeats and beat Tottenham Hotspur at Wembley on Saturday night, as they outclassed Mauricio Pochettino's side in every way.
United's defeat crowned City as EPL champions for the third time, their fifth title in England's first division league and the first under Spanish coach Pep Guardiola, reports Efe.
Manchester City now has five Premier League titles.
United manager Jose Mourinho asked for consistency from the France worldwide this week following his two-goal performance in last weekend's win over Manchester City. Everything was slow. Everyone was one more touch, one more trick, one more flick, one more turn. "Probably because I won eight championships, I was not in the moon with the victory against them because that doesn't make me feel super happy".
Mata knows how significant it could be if this year's team win the trophy in the coming weeks, even if Spurs represent hard opponents at Wembley.
"The mental level, the day by day, everything must be consistent until the last day so one of the things we have to improve is consistency", he said.
As for City, it has a month to finish a season with the most points, goals and wins in a single Premier League campaign. "We are frustrated ourselves with the performance and what we have to do is, from tomorrow, think about the next game and keep going, because we have very important weeks ahead". If Jose Mourinho-coached Man United can win all of their remaining matches, they can reach maximum 86 points in total.
Something tells us City will get there. West Brom are a good team and they don't deserve to be that low and as they stuck into the game you realised that something could happen.