He plays with top quality defensemen on a team that is fantastic at transitioning pucks out of the defensive zone and generating offensive pressure. I don't think it's unreasonable to wonder how he will look on a team that is routinely caved in its zone. FWIW, I think those are legitimate concerns with Cirelli and Sergachev too. It doesn't mean that they're not contributing to Tampa's success or that they're passengers or anything, but any time a player goes from one of the best teams in the league to one of the worst, you have to expect their performance to take a hit.
I mean, when is that last time a defenseman stepped on the ice for Tampa and looked bad? Stralman, Shattenkirk, and Bogosian are all very recent examples of defensemen that looked terrible either prior to or after playing for Tampa, but looked absolutely fine, if not downright good playing for the club. It's why I'm very reluctant to give up a good asset for any one of their RFAs. Happy to take an asset to help them move a contract (not TJ unless the price is huge), but the last thing we need to do is give up a good asset for Cernak and have him come in here looking like Mirco Mueller or something.