This isn't the Bertuzzi of years' past. He was .76PPG with Boston and instantly clicked in that system post-trade. Whether it's his back injuries that have caught up to him, or just a poor fit, I have very little confidence that his game is going to magically flip a switch come playoffs. Plays are outright dying on his stick and he can't keep up.
I would not be overly concerned with replacing a ~35pt pace impact for 5.5m. Robertson is not a solution there, but for comparisons sake, he's matching Bert's pace for sub 1m without playing with the elite core 4. You could partially address the D (i.e. a #4D) in a lateral move involving Bertuzzi and then later acquire a warm body to replicate what Bertuzzi has brought.
It comes down to what is more important - hanging on to this mythical idea of what Bertuzzi might be in the playoffs (which he's shown none of thus far), or addressing the blueline and making better use of that 5.5m + Klingberg's LTIR money. The alternative is spending a 1st ++ which I don't think we should entertain.