BOSTON -- Craig Kimbrel was supposed to spend Monday soaking in the All-Star festivities in San Diego.

Instead, the Boston Red Sox closer will be in a hospital.

Kimbrel will undergo surgery to repair a medial meniscus tear in his left knee and is expected to miss three to six weeks, the Red Sox announced Saturday. In his absence, Koji Uehara will be the Sox's primary closer, manager John Farrell said, with newly acquired reliever Brad Ziegler getting save opportunities when the 41-year-old Uehara is unavailable.

"It's the first time I've been on the DL in my career, so it's obviously frustrating," said Kimbrel, who has a 3.55 ERA and 17 saves in 34 appearances. "I'm kind of glad it's nothing too serious. It's something that I feel like we can go in, get it scoped out, get it cleaned out.

"They're saying three to six weeks. Let's hope it's more on the three-week side, if not a little faster than that."

Toronto Blue Jays right-hander Aaron Sanchez was chosen Saturday to replace Kimbrel, a five-time All-Star selection, on the American League All-Star roster.

Regardless, it was convenient the Red Sox were already looking for another late-inning reliever before Kimbrel got injured. Early Saturday morning, they acquired Ziegler from the Arizona Diamondbacks for a pair of Single-A players. The timing of the trade was coincidental, according to president of baseball operations Dave Dombrowski, who said he had been talking to Arizona general manager Dave Stewart for several weeks.

Craig Kimbrel will undergo surgery to repair a medial meniscus tear and is expected to miss 3-6 weeks. Mark L. Baer/USA TODAY Sports

Ziegler, a 36-year-old right-hander with a submarine delivery, has a 2.82 ERA and 18 saves in 36 appearances for the Diamondbacks. Once Kimbrel is healthy, Ziegler figures to take over the tough-on-righties setup role vacated when Carson Smith was lost to Tommy John elbow surgery in May.

"I think I made a comment to our own people that it'd be nice to get this done today because, with the Kimbrel situation, the timing would work out. But it really was just a coincidence -- a good coincidence to get him and bring him in here at this time," Dombrowski said.

"Ziegler's an established big-league pitcher at the back end of the pen. He's a different-look guy for us. He's a guy that doesn't throw hard but is a ground-ball machine."

Kimbrel said he took a step to field a ground ball in the outfield during batting practice Friday and "heard a pop" in his knee. He went for an MRI exam that revealed the torn cartilage.

Initially, the Red Sox medical staff held out hope that Kimbrel might be able to avoid surgery. Moreover, Kimbrel suggested he would be able to pitch through the pain.

"It was apparent to me in talking to Craig after [Friday night's] game and getting the medical information, this is not something you want to take a chance on," Dombrowski said. "You don't want Craig Kimbrel to go out there and alter his delivery and be in a position where he hurts his knee more but also hurt his arm."

Said Kimbrel: "I felt like I could go out there and pitch and kind of go through the pain. What it is, it's something that I wouldn't say is structural. Looking where we are in the season and what could come of it going out there and trying to pitch through it, it might change my mechanics or put more stress on my arm. We're just looking to take the safe route on this."

Kimbrel is the second Red Sox pitcher this season to get injury while retrieving a ball in batting practice, a routine that he and many other pitchers use as a cardiovascular exercise. Left-hander Eduardo Rodriguez also injured his knee during a spring-training incident.

Farrell said the team doesn't have any plans to alter their pitchers' pregame routines,

"This is an anomaly," Farrell said. "This is what our players go through normally, day-in, day-out, traditionally in the game. We can't put guys in bubbles. You get a little bit of a change in direction, and this is what happens. Unfortunately, in Craig's case we're going to miss him for a little bit."

Ziegler is scheduled to arrive at Fenway Park midway through Saturday's game and won't be activated until Sunday. Until then, right-hander Noe Ramirez was called up from Triple-A Pawtucket to take Kimbrel's spot on the roster.

The Red Sox also activated newly acquired utility man Michael Martinez and optioned rookie infielder Marco Hernandez to Triple-A. Martinez was obtained Friday from the Cleveland Indians for cash considerations.