Phillips injured his thumb diving for a ground ball in the eighth inning Wednesday night and an MRI on Thursday morning revealed torn ligaments. The four-time Gold Glove winner is hitting .272 with seven homers and 40 RBIs in 86 games.
The Reds put Phillips on the 15-day disabled list Friday.
"It is a big blow. Brandon is a big two-way player. He has hit in the middle of the order most of the year. Defensively, he has been spectacular. It is a blow, but we'll weather it," Reds manager Bryan Price said.
Phillips, 33, was moved down one spot to third in the batting order when Joey Votto missed 15 days earlier in May.
The Reds purchased the contract of infielder Kris Negron from Triple-A Louisville on Thursday, before they got the results of Phillips' MRI.
The team placed Votto on the disabled list earlier in the week with soreness in his left thigh.
The Reds' 37 errors are the fewest in the major leagues. Phillips made just one error in 378 total chances, fifth in the National League.
Zack Cozart, the Reds' shortstop, will be affected most by Phillips' absence.
"It is going to be tough for our team," Cozart said. "Brandon and I work well together up the middle. Injuries happen. Joey's [Votto] hurt. We have to figure out a way to win."
Cincinnati on Friday also bolstered its bullpen for the start of a weekend series with Pittsburgh, calling up right-handed relievers Carlos Contreras and Curtis Partch. Contreras was optioned to Triple-A Louisville on Thursday and brought back a day later. Partch is in his third stint with the Reds.
Catcher Tucker Barnhart was optioned to Louisville.
Information from The Associated Press was used in this report.