Elias Pettersson's 5v5 individual shooting percentage is currently aligned with the rest of his career
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Now, it is fair to say that Pettersson may experience slight 5v5 points regression due to oish%. However, he also has had very low luck on the power play which could also counter the 5v5 points regression when his PP pts regress back to the mean. So all in all, I am not convinced about this "huge regression" you are talking about.
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You are looking at raw on-ice metrics to be fair. When looking at any analytics that attempts to isolate one's on-ice impact from his teammates. Pettersson fairs a lot better and rapm numbers from both EvolvingHockey and Jfresh indicate that Pettersson does indeed tilts the ice.