Okay, I took a look at the schedule, and nearly half the schedule (26 games) is comprised of games that are the second night of a back-to-back, or the third game in a four day period. There's also one back-to-back-to-back.
How many of those games are the Celtics going to win? I think 13 would be very optimistic. I'm going to guess 8. So that's 18 losses accounted for already.