The injury forced Gasol to exit early in the Lakers' rout of the Mavericks on Sunday. He was examined by Dr. Steve Lombardo on Monday. An ultrasound exam and an MRI showed that Gasol has a "mild-to-moderate strain of the left hamstring," according to the team.
After the Rockets game, Gasol is listed as day to day.
Following Sunday's game, Lakers coach Phil Jackson said Gasol had complained of some tightness before tip-off.
Jackson, however, was a little more coy at Monday's practice when talking about how Gasol got hurt.
"It happened before the game, in a very unusual way, and I'll let Pau explain that to you when you see him," Jackson said.
The only hint Jackson provided was it being "something that's not related to exercise, in this regard."
Gasol, who missed the first 11 games of the season with a strained right hamstring, left with 4:48 left in the first quarter on Sunday.
Information from ESPNLosAngeles.com's Arash Markazi and Andy Kamenetzky was used in this report.