The New York Knicks have listed Stoudemire as day to day and Billups as questionable for Game 3 of the Knicks' first-round series with the Celtics on Friday night at Madison Square Garden.
The Knicks trail Boston, 2-0.
Stoudemire sat out of the second half of the Knicks' 96-93 Game 2 loss to Boston on Tuesday with back spasms. The Knicks' All-Star power forward/center said after the game that he aggravated the injury while attempting a dunk during warm-ups.
Stoudemire said after Tuesday's game that he will be ready to play Friday.
"I should be ready for Game 3," he told reporters in Boston. "We'll see how it goes [Wednesday] and next day, but I'm pretty sure -- I'm hoping -- I'll be ready Friday."
Stoudemire said he tried to play through the sharp pain he felt in his back but the team ruled him out after halftime. He finished with four points and five rebounds in 17 minutes.
Stoudemire, who had 28 points and 11 rebounds in the Knicks' Game 1 loss, was in so much pain that he had to stand during his postgame news conference. He also needed a team trainer to help him put on his socks and shoes after the game.
"He should be OK Friday. ... He just felt a little twinge," coach Mike D'Antoni told reporters in Boston on Tuesday night.
Shawne Williams replaced Stoudemire at power forward in the second half on Tuesday as Carmelo Anthony single-handedly kept the Knicks in the game. Anthony tied a career playoff high with 42 points and set a career playoff high with 17 rebounds.
Stoudemire did not play in three of the Knicks' final four games because of an ankle injury. He also sat out of New York's road win over New Jersey with a sprained big toe.
Billups sat out Game 2 with a strained left knee. He originally suffered the injury on a layup attempt late in the fourth quarter of the Knicks' Game 1 loss. D'Antoni has previously expressed optimism that Billups would return for Game 3.
Billups sat out of six games in early March with a left thigh bruise that affected his knee. He also missed the Knicks' regular-season finale against the Celtics. The Knicks are 4-5 without Billups. They are 2-2 without Stoudemire.
Ian Begley is a regular contributor to ESPNNewYork.com.