Moneypuck disagrees that Mantha was invisible using their expected goal model. The Ovi-Back-Mantha line generally controlled the majority of shot share while they were on the ice, especially Mantha, finishing with the best xgf% of any Capitals skater this game, a lot of it due to him not allowing many shots/shot attempts while he was on the ice, even if Wilson was more creative offensively with his scoring chances compared to Mantha.
The line still had their share of offensive chances as well, edging the Bruins in shot attempts 17-7 at 5v5 whenever they were on the ice. I wouldn't say Backstrom or Ovi were 'sheltering' Mantha either, he seemed to be controlling the puck well along the boards, read the play well, but never was the dynamic 4-goal power forward version of him that is seen from time to time.
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Matched up pretty well with the eyetest too, Mantha had some very rough nights defensively with Detroit this season where criticism was generally pretty deserved, but he notably kept the Bruins forward lines' shifts short for the most part whenever they were in the Capitals zone from my initial viewing.