AUBURN HILLS, Mich. – When Sam Cassell signed as a free agent with the Boston Celtics on March 4, little did the veteran guard know that it would lead to one of the most frustrating experiences of his 15-year career.

Cassell came to Boston as more than just insurance for starting point guard Rajon Rondo. Cassell signed with the top-seeded Celtics with the notion that he would be a key contributor off the bench.

He came to Boston a veteran of 115 playoff games, including two championships with the Houston Rockets.

But after appearing – and contributing – in the Celtics’ first 12 playoff games, Cassell took a backseat to guard Eddie House, the team’s spark plug.

“It bothers not just me, but everyone around, like, “Why are you not playing?’ ” Cassell said. “I’m not disguising it, but it’s just tough. I ain’t going to lie. It was tougher because I knew I could play. I competed against these guys with my teammates, with other teams before in the playoffs and had a lot of success. But me getting here late in the year, who knows?”

Cassell finally got back onto the court in Game 3 of the Eastern Conference finals, finishing with five points in an eight-minute stretch of the first half.

Cassell was part of a reserve unit on which four players combined for 23 points in Game 3.

He made the most of his minutes, proving he still has something left in the tank in his 38-year-old body.

“It was nice to get him back in this series, just for the fact that he can handle the ball, he can handle the pressure that they’re putting on us, and that’s nice for our second unit,” Boston coach Doc Rivers said. “We needed him in this game so our second unit could run their offense.”

Cassell’s size – 6-feet-3, 193 pounds – and experience make him the ideal candidate to defend the Pistons’ Chauncey Billups, one of the more physical point guards who likes to exploit mismatches with smaller defenders by backing them into the post.

But with Billups not playing at full strength, that situation has yet to materialize.

Still, with the series tied at one game apiece heading into Game 3, Rivers thought it was necessary to have a veteran guard on the floor early in the game. Cassell didn’t disappoint, although it took him a few minutes to get settled.

“This is not that tough, you know what I’m saying, compared to what I went through early in my career,” Cassell said. “It should be a grappling match, a fight getting the ball up the court. It’s not that tough.

“But I think the game has improved. But as far as jumping and running, the physical aspect of the game has definitely took a step backward, a major step backward. I don’t think the game is physical like it used to be by far. But I think that’s why I’m playing so long, too-the game is not that physical.”