I actually thought of this the other, but there are a few things to consider:
If the Celtics were in love with Capela they would have taken him at #17. If they liked him a lot but not as much as Young, they could have traded back in to get him.
Capela was a polarizing prospect. I've seen him ranked as high as #4 (seriously) and as low as #35. It's possible that the Celtics rate him low and Houston is in love with his upside + the fact that they can stash him for year or so.
Lin + Rondo + Smart + Pressey is way too many point guards. I understand that Lin's contract is really just a payment for Capela, but that's a steep payment unless you want to play the guy. Lin does bring a lot of energy and is a fun, albeit frustrating player. This would make more sense if Rondo is traded elsewhere.