Rooney has re-joined Everton after 13 years at Old Trafford that saw him become the club's all-time record goalscorer and win every major club honour.
But the 31-year-old started only 15 Premier League games last season and scored five times following Jose Mourinho's arrival.
Rooney found that situation tough to take, even though he tried to remain positive due to his role as United's captain, and feels he needs to play regularly to perform at his peak level.
"It was obviously frustrating," Rooney said of his lack of regular game-time at United.
"It was the first time really in my career where I didn't play. It is not me – I need to play football.
"It was frustrating but I was the captain so you need to keep yourself positive around the place.
"It hadn't gone stale. I enjoyed my time there but I needed to play more games, I didn't play that often last season and I know I'm at my best when I'm playing consistently.
"It was tough but that's football. Some people will accept playing every few weeks.
"For me, I love playing and that's what I want to do. I spoke with Jose and it was the right time for me to go and play somewhere else."
Rooney will not have to wait long for his return to United with Everton as the two teams are scheduled to meet at Old Trafford on September 17.
"I'm going there to try to get three points with Everton," he said. "I had a great time at Manchester United but football moves on, players move on and I have come back to Everton."
The forward insists Everton are his focus, but hopes his form makes it hard for Gareth Southgate to continue leaving him out.
"To be honest I'm focused on Everton and doing well," he said. "If I get back playing, Gareth Southgate will have a decision to make.
"If the performances for Everton are good enough he won't be able to ignore me."