De Niro was in the flashbacks, but I dont see what the problem is with them not sharing screen time.
It's a bit like Robert De Niro and Michelle Pfeiffer. They were in Stardust but had no scenes together, in fact a couple of other films as well – something that she was very pleased to rectify when they did The Family. It's like if you're going to be in a movie with Robert De Niro you at least expect to be in a scene with him .