"If there was one challenge, I would love the club to win more European trophies," he told CNN.
"I don't think we've won the number we should have won.
"There have been years where we were unlucky. The years we were disappointed. The years when we've thrown games away and that sort of gives you a kaleidoscope of what football is like in a way.
"So, therefore, if we could just win a couple more European trophies then I would definitely quit."