It is a conundrum. If you want to keep the Granlund line intact, you have to either play Smith on the third line with players not skilled enough to get the best out of him or play two rookies together on the second line.
Personally, I don't give a damn about the Granlund line because he's not the future of this team. Have Celebrini center your first line, Smith the second, and put some combination of Granlund, Toffoli, Eklund, and Zetterlund on their wings. Live with the growing pains. These two are going to be centering our top two lines for a long time and there's no time like the present to start developing them as such. Trade Granlund at the deadline and bring something back under team control. Ideally IMO a prospect who's ready for AHL/NHL time that can help out either your two young centers or your defense.