New challenges? What other club would he be a starter at (which he will insist on being) and win more trophies at? Maybe he should step up for England, that's a challenge he's never really succeeded at.
I can't explain 'new challenges' any more? He's won everything at United so he may want to try and win trophies elsewhere.
And what do you mean where else would he be a starter at? He'd get into most team in the world and certainly every team in the Premier League. He's not exactly a 'starter' at United either judging by his Premier League appearances this season.

