For those that are curious about these things, and on why Carlos Arroyos picked #45, here's the story.
Number 45 was previously worn by another Puerto Rican player who played for the Celtics in the 1988-1989 season, his name was Ramón Rivas. From what I understand, he was the 2nd Puerto Rican to ever play in the NBA. To many, he's considered the first solely because he was the first to be born and raised in Puerto Rico, while the one before him was raised in the United States.
He only had that one season in the NBA, and it seems like he wasn't successful in this stage.
So there you have it, #45 in honor of a former Celtic breaking barriers for the small island. Ramón Rivas is currently a Spanish broadcaster for the Magic.