I don't see any reason why Pressey will be gone. Cheap, good back-up pass-first PG. Young, good back-up pass-first PG. I expect he'll be here longer than Rondo.
He's on an unguaranteed contract, which makes him fodder to be shipped out in a possible trade.
Pressey doesn't seem to have potential other than as a backup and it is inexpensive to get rid of him, so he is a very likely casualty of any roster moves the Celtics might make in the off-season if the Celtics need to clear a roster spot.
I also expect Ainge to kick the tires on an undrafted point guard and look for an upgrade at the third-string spot.