When Koeman has his agenda against players he has them and won't give them a minute unless he absolutely has to, that's just how he is. Demir getting the first team number even tho it says in Collado's contract he must have one makes it perfectly clear, Demir could easily just play through the Barca B number.
As such Koeman must think he's really bad as even though we have a positional shortage after several forwards have left and the ones we have are far from a top level, and that it says in his contract he should be registered with the first team, he still kept him with the subsidiary number.
One of Collado's great strengths in set pieces (I think 5ga last season in Barca B were free kicks and corners) isn't very useful at a club like Barca where you have so many good set piece takers. He also wouldn't get near the same amount of the ball as he does in B and certainly wouldn't conduct the offense. 'A good player for a bad team, and a bad player for a good team'.
Due to his weakness in things like defensive contribution, chance creation, making runs, I can see why Koeman does not like him.