Everton brought Rooney back to Goodison Park on Sunday on a two-year deal, ending the former United captain's 13-year Old Trafford career.
The last of those honours came in Rooney's final United appearance as Ajax were beaten in the Europa League final, but the forward says he already knew he would be leaving when celebrating that triumph, having been restricted to 15 league starts last term.
Rooney explained: "It was just a frustrating time. The hardest thing was lifting the two trophies – the League Cup and the Europa League.
"You don't feel like you [deserve it] because you haven't been part of the game. That was hard to do; you're naturally happy because you have won but you don't celebrate it as much.
"It is just frustrating if you are not playing. The lads who play were having two-day recovery sessions while all the lads who were on the bench and not in the squad were training together, so you're not really training with the first team.
"It was around November I knew [an exit was likely].
Everton (@Everton) July 10, 2017
"I spoke to Jose in January to see what his opinions were and he always said he wanted me to stay and help the team until the end of the season. I did that.
"I helped in the games he put me in but I knew I had to leave to move forward for my career. It was a sad moment because I had been at the club for 13 years but I had to think of my career as well."