I wouldn't say he was useless. He brought elements that they wanted which was size along the boards, strength with the puck, and some speed through the neutral zone but it's clear he wasn't totally comfortable in the system yet. He was unlucky a few times or else he would have been on the scoreboard, and I believe they played him on PP1 when Kuznetsov was suspended, so it's clear the coaches saw value in him.
I think fans generally underestimate how difficult it can be for players to learn new systems in hockey. It's the reason that most deadline acquisitions turn out to be busts. It's why a lot of FAs struggle the first half of the season with their new teams. Some guys adapt quicker than others but all players go through an adaptation process. This isn't a video game where you just plug a player into a roster and everything is fine right away.