The midfielder has been a revelation for Chelsea this season.
Ahead of Saturday's FA Cup semifinal against Chelsea, Guardiola said: "He's an exceptional player, we knew when he was here. He's a nice guy, a shy guy, with a lot of confidence, and he's playing fantastically well.
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"But he was asking for two seasons to leave. I said 'no, stay'. But he still said 'no, I want to leave'. What can I say?"
On facing Chelsea after this week's Champions League exit, he added: "We don't have an option, we can't feel sorry for ourselves.
"We don't have time to reflect, we'll do that in the summertime. You compete to win, but sometimes you lose games, and that's what happened."