Well, that's all a load of bollocks
You've made it sound like GT is being held back by having to baby-sit Sheehan. GT's poor touch, poor passing and often ridiculous bookings is what is holding him back. Hopefully that will improve as he gets older.
Of the current crop of players only Sheehan is the one I'm confident in receiving the ball from the keeper with an opponent up his arse. That said, if SS is happy for him to go then so am I. He'll know more than any of us which players we need to keep and those that can move on.