Reilly is OK defending line rushes, and the front of the net. Not the most physical, but otherwise has the tools. Where he is at fault is when he tries to do too much in his own zone when he has the puck either by carrying it too long or by making risky passes. He also tends to make risky pinches in the O zone, and stay up ice too long on rushes, leaving his D partner facing an odd man rush should the puck turnover.
All of these things are judgement issues, not physical issues. They can be fixed, I think, with patience, and good coaching. Every time he got TOI in MN he tried to "make things happen", rather than let the game come to him. Understandable, as he had Brodin and Suter playing ahead of him, was fighting two others for 3rd pairing minutes, and felt he had to get noticed in order to play more.
He is not a meat and potatoes 3rd pairing Dman. If you expect that you will be constantly disappointed. He is Subban lite, basically. Remember the early years when he would make a great play, then an awful one? That's sort of where Reilly is right now.