Looking a bit closer at Puustinen's shift cart (
Time On Ice Report Home Team) I think he's being undercut by his linemates and by Sullivan's usage of the fourth line.
The shifts were normal in the first period. Six shifts, 4:01 total ice time.
The second period had five total penalties - three against the Sabres, two against the Penguins. That's 10 minutes of game time where Puustinen isn't used. So he only got one shift. Eller and DOC are penalty killers. They need a moment to catch their breath after the PKs. And Eller is on the second PP unit. It throws the lines off.
Puustinen got two shifts in the third, but you had an extended 5-on-3, a 2 minute PK for the Pens, and then an extended push at the end.
He's getting shafted a bit by game events, but that means Sullivan needs to find a way to spot him some extra shifts.