I'm probably in the minority these days, but I don't think there was a time when he was in a Celtics uniform when Antoine *wasn't* a good player.
He was a very frustrating player, in that he took too many bad shots and didn't rebound like his talent suggested he should. He didn't dedicate himself to staying in shape, and the "veteran all-star" bit rubbed a lot of people the wrong way. Still, he was a very effective -- if inefficient -- player here for the most part. I mean, he averaged 22 / 9 / 5 and helped lead his team to the Eastern Conference Finals, so the guy obviously did his job. I think he suffered from 1) poor teammates around him (other than Paul) and 2) weak coaching.
Once he left Boston, Antoine became even lazier and out of control. He lost a lot of his athleticism, and it was unacceptable to see the way his FT% declined. He was a big contributor on the Heat championship team, but after he got his ring, his game just seemed to fall off all the way. Once his athleticism declined, he was no longer the dynamic "point forward" of his early years, and simply became a poor shooting big man.