Saito was diagnosed with a sprained ligament in his pitching elbow Tuesday by a team doctor.
The 38-year-old right-hander will be placed on the 15-day disabled list when the Dodgers return from the All-Star break Friday night in Arizona.
He will rehabilitate in Los Angeles for an estimated six weeks before being evaluated.
Saito is 3-3 with a 2.18 ERA in 39 appearances. He has 17 saves in 20 chances.
Saito, an All-Star last year when he had 39 saves, was forced to come out of Saturday night's game against Florida because of what he called abnormal tightness in the elbow. He had an MRI on Monday.
Manager Joe Torre said Sunday that Jonathan Broxton would be the team's closer. But Torre was unsure who would take over for Broxton as set-up man. Among the possibilities are left-hander Hong-Chiu Kuo and right-hander Chan Ho Park.
"Kuo is the first one that comes to mind, to me," Torre said. "After what I had heard and what was documented about the [four elbow] surgeries he's gone through and wondering if we should start him or if we weren't going to be able to start him because we had enough starters, how is he going to respond coming out of the bullpen? But he's really been a horse."
Broxton, 2-2 with a 3.40 ERA in 40 games, said he's ready for his new role.
"I just look at it like it's the eighth inning. I come out and get outs," said the hard-throwing right-hander, who has 46 strikeouts in 39 2/3 innings. "It's no different for me."