Well, it is the regular season. So it's not that big of a deal if one of the rookies had to do that for a few weeks until one of them made it back.
Another option, of course, would be to give PP some minutes there. At this age, I wouldn't want him to play 25 mpg there; however, I could see a scenario where he plays 12 mpg at the 2 and 18 mpg at the 3. That would leave 30 mpg for Green at the 3. It would also mean that we'd only have to fill about 15-20 mpg with a rookie.
Again, not ideal, however, assuming that 3 out of the 4 are always healthy and that no one faces a season ending injury, we should be fine. And if someone does face a season ending injury, Ainge will likely then look to pick up a Daniels (or better yet a Pietrus) who can step in.