I don't see any reason why if Rondo is ready to play one minute, why he wouldn't be ready to play 30 minutes. He had a broken finger. Nothing there to prevent conditioning or anything like that. I don't see Rondo coming off the bench. If he plays, he should start and be prepared to go all out (which I am sure what Rondo expects too). If he is "rusty" work the rust off in practice.
From both Rondo's perspective and the team's perspective, Rondo needs to be on the court. Rondo wants a big contract and purportedly wants to be "courted" as a top free agent. To get that, he needs to be on the court.
The Celtic's want one of two things, but both are enhanced by Rondo playing well. If they want to trade him, it is obviously better if Rondo is playing and playing well. If they want to resign him and build around him, then having him on the court and showing that the team can be half decent would validate this point.
And if he breaks his finger again, so be it. It is a finger. Now if Rondo is not ready to play period, then of course, you don't put him out there but as I said, it seems that if the finger is ready to play a little, it would be ready to play a lot.