It's entirely Motta's fault.
Do you guys remember when Nedved arrived at Juventus? His first season was really bad, because the "jump" from playing between Lazio to Juventus cost him a lot.
Koop is experiencing exactly the same thing, even more difficult.
Koop is in his first season in a big team, with a new coach, with a new formation and in a new position and Motta, instead of helping him in this transition, has rather made it worse for the player.
Motta, being a coach with little experience, has tried to give "confidence" to Koop by not wanting to replace him, by going to the press to say that Koop is "irreplaceable" and even more so by giving him the captain's armband and all these things instead. To give the player confidence, he has ended up putting much more pressure on him, and Koop has not been able to handle this pressure.
An experienced coach would have noticed this situation and would have reduced the pressure on the player by leaving him on the wing for a few games, or using him from the bench.
Motta trying to "help" Koop has rather harmed him, brind down the pressure for him, sit him for a couple of games or use him from the bench, its not like we dont have options to play in his place, Motta needs to understand this and when he does he will help Koop and then probably we will see the player we want.