JJ had good stretches of play in previous game and a couple of good saves. He is very shaky, however and has big issues in fundamentals like positioning his big body with big equipment wrapped around it and tracking the puck. The first problem is fixable, I don't think the latter one will be.
He is so big though, that he will be a decent backup if he just learns to position himself properly and set those pads correctly. He should watch Rinne, for an example, of how a big goalie plays and learn from there about positioning yourself. His movement seemed ok while on skates, but he looks very awkward many times in setting himself between shooter and the goal. Good game yesterday, anyway.
Avs are most likely going to face Golden Knights during playoffs in second round regardless of who wins the division. I'd rest Grubauer as much as possible and let JJ play whenever possible unless he starts performing like Miska.