Reddick, who went on the DL on Wednesday, had been bothered by discomfort from a skin infection near his left knee since discovering it on the team's flight home from Anaheim on May 16.
Reddick said his best guess was that he was bitten by a spider.
"It still pretty gross-looking," Reddick told SportsTalk 790 on Tuesday. "But the pain is a lot more tolerable, and the level of the pain has gone down severely. ... It's slowly getting there, just a very unfortunate accident that never really happens a whole lot."
The right fielder tried to play through it, but had since experienced increasing soreness. Reddick is hitting .227 with six home runs and 18 RBIs this season.
"He just hasn't conquered it," Astros manager A.J. Hinch told reporters Wednesday. "It's extremely sore, and he needs some medical attention. He needs some time off, and we're not going to play with a short roster."
To fill Reddick's spot on the roster, the Astros recalled Jake Marisnick, who was demoted to Triple-A Fresno on May 15 after starting the season with a .141 batting average and 41 strikeouts in 85 at-bats this season.