Maybe Ainge drafted Romeo because Romeo had the most potential and could be used as trade bait later.
They drafted him because they thought he was the best player available. This is a pretty common practice.
Of course, you don’t know what’s going to happen to the guys you already have, so by the end of Romeo’s rookie contract he could either replace someone ahead of him (because of injury, or because the guy is traded, or even because he beats him out) - or be traded himself. The beauty of a rookie contract is that you don’t have to know which outcome in advance.
Looks like he’s got a lot of ways to score off the dribble; and that really nice burst he has on the drive serves him well as a wing defender as well.
I don’t believe that there is such a thing as an injury-prone player - though it is true that if a guy gets hurt enough he can be permanently diminished.