DUNEDIN, Fla. -- Blue Jays closer Casey Janssen will miss the start of the season with a left abdominal and lower back strain.

Toronto placed its veteran right-hander on the 15-day disabled list on Sunday afternoon. The move is backdated to March 29, which means the earliest Janssen will be available this season is April 13 vs. the Orioles.

Janssen had been bothered by a sore right shoulder all spring, but this is the first time he's dealt with a back issue. Janssen suffered the injury while warming up during Friday night's game against the Mets in Montreal, and the discomfort lingered into the weekend.

"We don't expect him to be out too long," Blue Jays general manager Alex Anthopoulos said. "There was a scenario where we could have waited five or six days, try to get back going, but we want to have this thing resolved and not nag at him. He's going to continue to throw to keep his arm in shape, and then once it's completely gone, we'll get him [throwing] off the mound, and we don't expect to lose him for too long."

The 32-year-old Janssen made just three appearances during the Grapefruit League season because of his shoulder injury. He was in a no-throw situation for the first several weeks of camp and had been in a rush to get ready for Opening Day.

Right-hander Sergio Santos will temporarily take over as the Blue Jays' primary closer. Santos was originally acquired prior the 2012 season to become Toronto's closer, but he battled a series of injuries before eventually settling into a setup role.

Santos saved 30 games for the White Sox in 2011 and has a career ERA of 3.21 in 145 2/3 innings. Right-hander Steve Delabar and left-hander Brett Cecil will likely handle the eighth-inning duties.

Janssen's injury has prompted the Blue Jays to go with a seven-man bullpen. The original plan was to start the year with eight relievers, but the injury caused the club to change course, as catcher Erik Kratz has been recalled from Triple-A Buffalo to take Janssen's spot on the 25-man roster.

The promotion of Kratz comes at least as somewhat of a surprise, considering it means the Blue Jays now have three catchers on their 25-man roster. It's also an understandable move, though, because Kratz gives Toronto a much needed right-handed bat off the bench.

The Blue Jays' reserves now include Kratz, catcher Josh Thole, infielder Maicer Izturis and outfielder Moises Sierra. Infielder Ryan Goins and designated hitter Adam Lind both struggled against left-handed pitching, and the addition of Kratz gives the Blue Jays a little more flexibility to use platoons or late-inning pinch hitters.

Thole is R.A. Dickey's personal catcher, but this opens up the possibility of the switch-hitting Dioner Navarro getting the start on Opening Day vs. the Rays and left-hander David Price.

"We could have gone a number of ways with it. Just because we're going this way, doesn't mean it can't change three days in, four days in, five days in," Anthopoulos said. "I know the focus is on the Opening Day roster, but the roster Opening Day can change the following day. It's really not that big of a change compared to the roster you'd have in the middle of April.

"We're going to start this way. We face three left-handers in our first seven games and having as many right-handed bat options makes the most sense now. We may change that five days in, a week in, two days in, if we decide we need an outfielder, a guy that can steal a base -- all those types of things."