Baggies striker Saido Berahino netted in each half and Chris Brunt added a third on the hour to humble to Premier League champions.
And Mourinho - who saw the perfect opportunity to take a dig at his rivals - has put the surprise loss at The Hawthorns down to the fact his side won the championship too early in the season.
"I cannot say it's a big surprise for me," the Portuguese said. "I know what normally happens when a team is champion with matches in hand.
"Bayern Munich lost the last three matches in the league, and when you're champions so early it's difficult to keep the motivation high.
"When you lose that salt and pepper of the competition it's difficult to compete at the highest level.
"If I have to blame anybody, blame Man Utd, Man City, Arsenal and Liverpool for letting us win the title so early!"