Gasol sustained the injury in Game 2 last Wednesday and tried to play through it in Game 3 two days later, but had to be substituted off after scoring six points in 22 minutes.
Gasol underwent an MRI on Saturday, which revealed the strain and as a result he was forced to watch Game 4 on Sunday from the sidelines, which the Cavaliers won thanks to Lebron James' incredible game-winner at the buzzer.
Bulls head coach Tom Thibodeau revealed that Gasol was feeling better, but added that the Spaniard is only doing rehabilitation work and has yet to take part in practice.
"[He's feeling] a little better today," Thibodeau told reporters after a practice session on Monday. "Day-to-day. I don't know [about his availability]."
Power forward Taj Gibson is expected to take Gasol's spot in the starting line-up once again after he scored six points and grabbed eight rebounds during Gasol's absence in Game 4.