Tough break for the Celtics but I don't think that changes that we are still a title contender. Not as much as before the injury, RWill is a good player, and impactful player, but we do have others (Theis and GWill) who will need to step up.
Incidentally, I was curious to see the 2-man results for various combinations of our bigs, essentially with Theis in for RWill: (these are normalized per 100 possessions):
Horford/RWill +13.6 51 games
Horford/Theis +33.0 7 games
GWill/RWill +4.7 56 games
Theis/GWill +13.2 13 games
The sample size with Theis is admittedly small but Theis is better than both Horford and GWill than RWill was with either. Again, small sample size and context but if you look only at the last 10 games:
Horford/RWill +21.5
Horford/Theis +32.9
GWill/RWill +20.6
Theis/GWill +14.9
Theis still pairs roughly as well with GWill and Horford as RWill does. Theis should also be getting better still, at least a little. Incidentally, Horford and GWill over the last 10 games are only +5.2, the worst combination.
We can overcome this. According to these numbers, Horford and Theis should play the most, Theis and GWill the next most, and Horford and GWill the least.