Seamus Colemole
Previously deathbyropeandglass
Thats not even up for dispute.
Gibson and Fellaini make us tick and both play superbly in their current roles.
Gibson is a decent central midfielder who has certain attributes. So when he plays in a midfield that so often previously lacked that he's bound to improve the team.
Its pretty simple really, you have a proper midfielder in midfield you'll play better than when you don't.