"It's the finals, man. You've got to suck it up and go all out," he said before practice at the Los Angeles Lakers training facility.
"I feel like I can go (tonight). The doctor feels the same way. There's a strong possibility I'm playing."
In the 30 minutes of practice open for reporters to watch, Perkins kept his left arm mostly immobile. He took some warmup shots, as did the rest of the team, but they were one-handed -- with his right arm -- with his left arm hanging at his side.
"It feels all right, mostly a little sore," Perkins said. "The doctors said I could do nothing in practice today. I'm going to give it some rest and give it a shot tomorrow. It's getting better every day."
Perkins left Thursday night's Celtics victory clutching his left shoulder after slamming into Lakers forward Lamar Odom early in the third quarter. The next day, Boston coach Doc Rivers said Perkins had a 50-50 chance "at best" of playing in Game 5 tonight.
But Rivers said Saturday that both Perkins and point guard Rajon Rondo, who had a bone bruise in his left ankle, would be in the starting lineup today, when the Celtics try to clinch the NBA championship.
Rondo spoke to reporters on Saturday without the hard plastic brace on his ankle he had been wearing.
"I think everybody's good," Rivers said. "(Perkins) is moving his arms and shoulders pretty well, so again, we won't know until tomorrow, but right now I think everything looks good."
Lakers coach Phil Jackson said the injuries didn't seem to hurt Boston in Game 4, when the Celtics came back from a 24-point deficit to win 97-91.
Eddie House scored 11 points in place of Rondo, and P.J. Brown had a big dunk in the fourth quarter.
"That turned out to be a benefit for them," Jackson said Saturday. "It turned out to be something that we weren't as adept at defending as we felt we could defend the other way around. And we should have been ready for it."