"You can not imagine", Guardiola said on Saturday before the title race was decided.
Mourinho also lashed out at his men for taking the victory over arch-rivals in the derby in the head and called the team "masters of complication" while shedding light on the shock defeat to West Brom.
Meanwhile, United manager Jose Mourinho makes two changes to the team that beat Manchester City in the last game, with Eric Bailly replaced by Victor Lindelof and Juan Mata coming in for Jesse Lingard.
The Spaniard also said that Saturday's defeat was a big disappointment after the second-half display at Manchester City.
"Congratulations to Manchester City on their Premier League title win.", Man Utd said on its official Twitter account.
The defeat put United's second spot position in danger, as their advantage over third-placed Liverpool was reduced to just one point.
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Pogba was substituted after 57 minutes of United's defeat and Mourinho did not make any attempt to hide his unhappiness with the team's performance and the inconsistency of his most expensive player. The Baggies have struggled to muster any sort of positive result of late and are unlikely to pull off a miracle like their did under Tony Mowbray years ago.
And United were unable to claw back the deficit, handing the Premier League to their neighbours when the final whistle sounded.
City have won the title with five games remaining, equalling the record set by United in the 2000-01 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", he said.
"It's incredible the way we played and I am so glad to be here and be part of this club", he said after the 3-1 win over Spurs at Wembley. They won the title because they were the best team, because they won a great amount of points.
West Brom had lost eight of their previous nine games and last won on January 13.
"We know that pressure comes with it, and that criticism comes with it, but we know what we have as a team and what we can do".