It was an extremely poor performance from United, who were booed off by their own supporters at both half and full-time. They created very few clear opportunities throughout and were punished with around 20 minutes left to play as Jay Rodriguez scored a stooping header for the Baggies.
Mourinho felt that his side allowed their opponents too much time to get into shape when they were on the attack.
Speaking after the game, the United boss said: "We were masters in complicated football. We could not play fast, we could not think fast, everything was complicated.
"We always gave them the opportunity to be solid and compact. They always had time to organise themselves."
He added: "The time of our best period was the time when they scored their goal. It was the only way they could score a goal which is from a corner or a free-kick."
City officially confirmed as champions
Mourinho feels that the Citizens fully deserve their crown and that it was always going to happen at some point.
"I congratulate them," Mourinho said. "I knew that sooner or later that would happen.
"If I was in their position, I would be very upset if people said that Manchester City won the title because Manchester United lost to West Brom.
"They won the title because they are the best team. They have lost only two matches because they are the best team and are champions."
In his post-match press conference, Mourinho made reference to the eight titles he has won in his managerial career and why this was a reason behind why he did not "feel super happy" when his side defeated City last weekend.
However, when questioned on how City compare to his Chelsea team which won the league a couple of years ago, Mourinho was reluctant to comment.
"It is time for them to speak about themselves, not for me."
Can United be more competitive next season?
United currently find themselves sat 16 points behind City with four games to go. This is not good enough considering the amount of money which has been invested at Old Trafford in the last couple of years.
Mourinho understands what is expected at the club and is confident that he has the ability to bring success in the future.
"I trust in my work, that is for sure. I win titles. My last Premier League title was not 20 years ago, it was three years ago. I believe in myself.
"You do not win titles with inconsistency, you win with quality. You have to be consistent at every level, not just the performance level. It is one of the things that we have to improve."
United have the opportunity to bounce back from this disappointing defeat on Wednesday night when they travel to Dean Court to take on AFC Bournemouth.