The Reds open a three-game series vs. the Brewers at Miller Park on Friday.
In 40 games this season, Phillips is batting .275 with seven home runs and a team-leading 34 RBIs. He is batting .350 in the month of May.
The injury happened when the Indians' Grady Sizemore hit a ground ball that hit the end of Phillips' thumb on his bare hand.
For the second game in a row, Jerry Hairston Jr. started at second base in Phillips' place against the Astros.
In other injury news, third baseman Edwin Encarnacion also hit off a tee Monday for the first time since he went on the DL April 28 with a chip fracture in his left wrist.
There is still no timetable for Encarnacion's return. He's been eligible for activation since May 13.
"It's been a long time for me," Encarnacion said. "I've never had to wait that long to swing. It's feeling a lot better. It's getting strong, and I'm working hard to get it stronger."
Left-handed reliever Bill Bray visited the Reds' clubhouse on Monday for the first time since having Tommy John ligament replacement surgery on his throwing elbow last Tuesday. Bray was sporting a large metal brace and wrap over his left arm.
The prognosis for Bray is he will need 10-12 months of rehabilitation with a target date to be back pitching competitively by April 1, 2010.
Before the surgery, Bray had started long-tossing and was working in his first bullpen session while coming back from what was originally deemed an elbow strain.
"I had gotten out to 120 feet with no problem," Bray said. "Something wasn't really wrong until I got into the bullpen two weeks ago. As soon as I got on the mound, I threw five pitches and each pitch after that started to hurt worse."
Bray, who has been beset by numerous arm injuries the past couple of seasons, had a good attitude about his situation.
"The more I think about it, the more I'm thankful for our trainers and physical therapists and Doc Kremchek [Tim Kremchek, the Reds' medical director]," Bray said. "Thirty years ago, I would have been done. It's a blessing we have the technology to keep me on the field even though I've been injury-plagued. I would have been done so long ago."