Rooney made 25 appearances in the Premier League in the 2016-17 season, when he became the club's top scorer.
But only 15 of them showed up at the start and said it was hard to play a bit, being a key player for almost all 13 years in the club.
United won the League Cup and the Europa League this season, but the money was exchanged at the final table of both finals.
And he claims that it has become embarrassing for him.
"I came for one minute in the Europa League final," he said.
"I was about to face Southampton in the League Cup final and at the moment it was embarrassing.
"It came to the fact that I was embarrassed.
"I thought," I can not keep doing this, "as hard as it was to leave United.
"In a game in Southampton Mourinho came to me and said:" I want you to raise the trophy. "
"I was like," Well, I did not even play this game. "
"He was persistent in this and literally picked him up and moved on.
"What have I been doing? I have to go and go somewhere else."
Rooney left Old Trafford at the end of the 2061-17 campaign to return to Everton in the boys' club before joining the MLS outfit at Washington DC United.
The 33-year-old retracted years in America and scored 12 goals in 20 appearances, helping them in the playoff phase.
He also made an emotional return to the England national team to win the 120th and final cap on Thursday's victory over the United States at Wembley.