Although McDonagh was probably our best prospect, Valentenko was most certainly not our 2nd best, as far as defensemen go. He's a big dude who plays a physical game, but you have to remember that he is currently in the KHL, and who knows if he will ever come back.
The thing is, Montreal still has some really good defensive prospects such as Yannick Weber, Matthieu Carle, P.K. Subban, and David Fischer.
The problem with them is they are all "offensive defensemen", whereas MCdonagh had a chance to be a 2-way defenseman