Gibson did not play in Wednesday night's 102-89 loss to the Orlando Magic and was examined during the game by team physician Dr. Brian Cole. Second-year big man Cameron Bairstow started in Gibson's place.
Gibson missed a game for the first time this season, and he acknowledged earlier Wednesday that the hamstring was "still hurting" after he injured it midway through the third quarter of Tuesday's loss to the Miami Heat.
"I hope it's not long," Gibson said of his absence. "Because I really wanted to try to play every game. That was my goal coming in. But it was just tough. My hammy was really bothering me for a long time, but I just kept playing through it, tried to play through all the fatigue and stuff."
The Bulls are already playing without Joakim Noah for the rest of the season after shoulder surgery, but they got some good news Wednesday about All-Star swingman Jimmy Butler. Butler has been out since Feb. 5 after spraining his left knee.
"[He] had a great workout today," Bulls coach Fred Hoiberg said. "I actually talked to him on the phone a little bit ago, and he'll be back in there [Thursday], practice fully with the team on Friday, and then we'll make the decision. It's still too early to tell if he'll play on Saturday, but he's definitely making progress."
Hoiberg also sounded optimistic that injured big man Nikola Mirotic's return isn't far away. Mirotic has been out since Jan. 28 because of complications following an appendectomy.
"He's making progress, had a good workout," Hoiberg said of Mirotic. "The plan is to hopefully have him do some dummy stuff in practice on Friday and then start ramping up some contact drills, some one-on-one, two-on-two, progress him up to some five-on-five scrimmaging. [He's] probably still a week and a half, two weeks away."
Mirotic said after the surgery that he lost 17 or 18 pounds, and Hoiberg admitted that Mirotic still hasn't gained all the weight back.
"The biggest thing is the pain and getting his strength and weight up," Hoiberg said. "That's going to take some time. Again, he lost 15 pounds after that surgery. [He's] still down some weight. The big thing is strength and I think confidence, knowing that he can take a blow down in that abdominal area and play in that area."