Sesar is a mediocre player. His fellow countryman and former teammate Danilovic is also a good answer to the OP question, btw.
It's curious nobody mentioned the ex-future-Celtics Nikos Galis. Would we have won more, less or the same number of titles during the 80s had he joined the team?
Also, I'd like to point out some Americans who never had a fair chance in the NBA - Clarence Kea, a very physical SF, with excellent rebounding skills and post up game; Michael Young, a Celtics draft pick and a tall guard with amazing outside shooting and very clutch; Kevin Macgee, a shooting undersized PF that had the bad luck of playing in a time where shooting and undersized PFs were not common; Audie Norris, another undersized post player, though as nails; Johnny Rogers, another shooting PF; Derrick Sharp, a shooting specialist who wouldn't do worse than the Houses of this world; Anthony Parker - he got his chance but too late, he was an elite perimeter defender during his prime; Johnny Rogers, an awkward but very productive player; JR Holden, a fast and skilled point. I predict Marcus Haislip to be the next one to make this list.