The five-time All-Star sustained the injury during Game 3 on Sunday, and he was listed as questionable in advance of Game 4. Bulls coach Tom Thibodeau confirmed that Gasol would miss Game 4 during his pre-game comments, but declined to announce a replacement.
"He's still sore," Thibodeau said, adding that Gasol hasn't gone through basketball work since sustaining the injury. "We've dealt with injuries before. Next guy get in there and get the job done. We'll see how it unfolds. We'll see how the game tells us what we need."
Gasol played just 21 minutes in Chicago's Game 3 victory due to the injury, finishing with six points and four rebounds. The Spanish big man played a key role in Chicago's Game 1 victory, finishing with 21 points and 10 rebounds.
Gibson and fellow reserve forward Nikola Mirotic filled in nicely for Gasol during the second half of Game 3, as Thibodeau opted to go small by removing Joakim Noah and playing Gibson and Mirotic together. That smaller lineup matched up well with Cleveland's undersized frontcourt pairing of LeBron James and Tristan Thompson.
"Whether we start Taj or Niko, they're all going to play a significant role," Thibodeau said.
Cleveland adjusted its starting lineup from Game 1 to Game 2, replacing shooter Mike Miller with the bigger, stronger Tristan Tristan Thompson at the power forward position. Cavaliers coach David Blatt stuck with his same starting lineup in Game 3—Kyrie Irving, Iman Shumpert, LeBron James, Thompson and Timofey Mozgov—despite getting guard J.R. Smith, his regular-season starter, back from a suspension.
Gasol, 34, averaged 18.5 points and 11.8 rebounds this season, his first in Chicago. The two-time champion was named to the 2015 All-Star team and is averaging 15.1 points and 9.9 rebounds during the playoffs.
"It's not a matter of who is playing, but how you're playing," Blatt said, when asked about the impact of Gasol's injury.
Gasol is considered day-to-day in advance of Game 5 in Cleveland on Tuesday. Chicago holds a 2-1 series lead entering Sunday's Game 4 at the United Center.