The Reds succumbed to a 1-0 defeat at the Old Trafford, thus, allowing archrivals Manchester City clinch the English Premier League title.
After that, West Brom used up the final half hour and got the win.
United was too narrow and predictable, with wide players Alexis Sanchez and Juan Mata constantly cutting inside and leaving the midfield congested.
United's afternoon was symptomatic of their entire season.
Newcastle virtually assured its Premier League status for another season by coming from behind to beat a depleted Arsenal side 2-1 thanks to Matt Ritchie's second-half victor on Sunday.
"We were masters in complication, everything was complicated".
"It's incredible the way we played, and I am so glad to be here and be part of this club", he said on the club's website.
Against West Brom, however, it was down to the home side to set the pace in front of their own fans, to establish a dominant pattern in the game.
It's the third Premier League success for the club after the titles won in 2012 under Roberto Mancini and 2014 under Manuel Pellegrini, with captain Vincent Kompany, David Silva, Sergio Aguero and Yaya Toure involved in all of them.
Manchester City were always going to be favourites this year, a no-nonsense turnover of players and signings such as Bernardo Silva and Kyle Walker were never going to make them any worse but Manchester United fans expected their team to put up a bit more of a fight. His players, in other words, have started choosing their games and, in the case of Paul Pogba in particular, it was not a surprise if Mourinho picked up the stench of complacency.
"I hope that we can have a happier Monday in seven days". Guardiola has won the League Cup already for City this season but a first Premier League title, to go with the three La Liga titles he won as manager of Barcelona and the three Bundesliga titles he delivered for Bayern Munich, will be particularly satisfying for the Spaniard. In sealing the league at this stage, City matched the record for the earliest title win, held by United since 2001.
"I think we were deservedly punished", he said.