Pretty sure Smith playing LW (like he did last year) was the contingency plan, and Kelly was brought it because he replaces Smith as a left-handed center or, if need be, bottom-six left-winger. Don't forget that we played without Clarke for pretty much all of last year, and were doing pretty well until Turris went out. Not many rebuilding (young) teams have the depth to overcome their top center and top left winger (he played primarily with Turris in the previous two seasons) going down at the same time. And even with all that, if we had Boro-Wideman as the 3rd pairing (not Boro-Cowen as the first PK unit) and Phaneuf in the lineup all year we still probably make the playoffs.
Again, I'll repeat, not ideal, and if we were not on an internal budget then you'd certainly want a better backup plan. But Sens still expect to make the playoffs even with MacArthur out of the lineup.