Footballers don't really get any other holidays throughout the year. Also, there is more and more talk and clarity of the reality of mental and physical fatigue that results from the constant pressure and burden of busy seasons, which was this year made worse by the EC. It's known to lead to injuries, which is the very last thing United need more of.Here is the thing. Every person needs holliday. No question about it.
BUT, sometimes there is a context around that. When you change clubs like he did, you could that year make a shorter holliday and start early on your new job. Not just because of your new employer but because of you too. To adapt quickly and be ready for first game.
Especially when that employer invested 40 mil in you and gave you big wage.
So I'd say better safe than sorry: let him have (and take) the full holiday he's entitled to, so you don't have to second-guess yourself in the season. (Is he fully fit? Mentally fully rested? Is that knock he has because he didn't rest enough? And so on.)