IMO, Celina is at least 2 years away from our first team squad so a loan to the Eredivise makes sense to me. He'll have some familiar faces with Unal and Yeboah, and if he does well City can look into a Championship/Premiership loan next year.
One under the radar benefit, is that that the Championship (and really the bottom prem teams) are extremely physical. At 18/19/20, players like Celina are still developing physically and it's not always the best idea to "throw them to the wolves", so to speak. Guys like Celina and Manu Garcia are also hardly giants on the pitch in physical stature, either. A loan to continent allows them to demonstrate and focus on their technical ability, while shielding them somewhat from getting kicked up and down the pitch and risking injury. If I had to hazard a guess that's a major factor in why we send so many of our U20 players (especially those not from the UK) are sent out to Holland/Spain/Italy/etc.