I guess it all gets shaken out in camp. I imagine one of Tatar or Johnsson will play on the fourth line as Wood probably won't be ready if he isn't even skating yet. If Zetterlund picks up where he left off and maybe improves a bit I'd expect him to outplay both Johnsson and Tatar. I agree Tatar could replace Boquist on a third line but hopefully Boquist improves as well from where he finished last season. The top six seems set absent another trade and unless someone else gets hurt there just isn't room for Haula to play top six minutes. Maybe he's a bit of insurance if Mercer slumps? I think Haula between Boquist and Zetterlund could be ok. They don't have to score a ton if they can just win the balance of the run of play and outscore the opposition by whatever amount they've done their job. If Haula scores 15 goals and 35 points or thereabouts and is on the plus side of the goal scoring and the fancy stats I'd think that was a great result.
Addendum: Obligatory reference to playing with Hischier on the third line.
