What the point of signing Djalo if we were not sure of his ability?
Djalo was a market opportunity, he would have left for a better club had it not been for his injury. It's really unfortunate situation for him, and short sighted.
Allegri was probably fine with bring Djalo, because he saw it as low risk, knowing he can slowly bring him in, but when the pressure rose Allegri stuck with his stubborn mentality of playing his favourite cards.
Motta isn't in the mood to experiment, he needs players who are ready, can handle the training and match practice, Djalo isn't able to do either and therefore makes sense for him to go.