Most of the comments I saw from Celtic fans online was that they thought he was a good player, but that other positions needed more immediate action, which is perfectly fair because they have quite a few midfield options
My first thought when they were looking at him was that it wasn't a great move for him because he'd probably not get into that team right now and it's much better for his development to get regular minutes. Let's face it, he wouldn't get much game time if he came here either
My opinion is that these younger players need to stay in Scotland and get some proper time playing regularly for a non Old Firm club. Too many young Scottish players get a move to the Old Firm or a club down south and end up not playing, which only hurts the National Team because progression gets stumped quite early on
The best outcome would be Turnbull playing at least two more seasons at Motherwell, but it looks like they want to cash in and he wants to move