I only see one option left, I'd rather Coyle at C but we have a beggers/choosers situation going on as far as goal-scoring talent goes in Boston these days, so got to make due with what you have.
Line 1
Marchand - Bergeron - Pastrnak
No explanation necessary
Line 2
Ritchie - Krejci - Coyle
Puts Krejci with the best goal-scoring left shot forward and best right-shot forward. Both bigger bodies. Sorry to say this but time is catching up to Krejci this year, he needs help and Coyle is the only guy I can see on the current roster who can provide the help he needs.
Line 3
Debrusk - Studnicka - Smith
Debrusk doesn't work with Krejci, Smith looks overmatched both offensively and defensively on the Krejci line especially with Ritchie as his winger partner. Studnicka is a better center, so is Coyle but Krejci needs help. Gives the line a veteran presence with Smith.
Line 4
Trent Frederic and friends
Whether Frederic is at LW or C, he should be the permanent fixture of this line. We'll call it the Frederic line. Take your pick of Kuraly, Wagner, Bjork, Blidh, Kuhlman, Kase, McKegg, Senyshyn, etc. They are pretty much all the same at this point, pluggers and/or question marks. I really don't care who the other two forwards they dress are, as long as T-Fred is anchoring the line.