I'm not sure I see the sense in Jarnkrok at centre. Hasn't consistently played there in a long while and his success with Seattle has come at right wing IIRC. Let's just say the ideal top 9 has been the lineup used w/ Ruzicka in between Mangiapane and Toffoli. There is only one real obvious hole in that top 9 and it can be plugged like:
Gaudreau - Lindholm - Tkachuk
Mangiapane - Ruzicka - Toffoli
Coleman - Backlund - Jarnkrok
And then the 4th line is swap-in city. Put Richardson in waivers ASAP, or at the very least get him back up following the deadline if a centre to displace Monahan from the 2nd line isn't snagged. But for the 4th line, you can play Monahan or Dube at centre (please Dube), then Lucic, Monahan, Ritchie, and Lewis can cycle in based on who's hot.
I think Lewis utility is greatly diminished with the addition of Jarnkrok. The PK groups can be: Backlund - Lindholm, Coleman - Jarnkrok, Mangiapane - Toffoli, and Dube as a spare. Whereas Ritchie has been alright on the 4th line and has been marred by a lack of scoring. I think he provides more than Lewis right now, for his role, given what Jarnkrok brings. Either way there are opportunities to work people in when tired and keep it as an energy 4th line.