CHICAGO -- Tigers reliever Bobby Seay will miss this weekend's series against the White Sox with a pectoral strain, but he's hoping to be back Monday.

Seay complained of discomfort around the front of his shoulder after Thursday's series finale at Cleveland and was sent to Detroit for an MRI. The results showed no structural damage, and head athletic trainer Kevin Rand categorized the injury as a mild strain of his minor pectoral.

Seay rejoined the team in Chicago Friday and plans to work out with the staff before he's re-evaluated Monday in Detroit.

"I'm shooting for Monday," Seay said after Friday's 2-0 loss. "I'm going to come in here and try to work out [Saturday]."

Seay said he felt much better Friday than he did when the injury occurred on his first pitch to Travis Hafner Thursday night.

The injury leaves the Tigers without their left-handed relief specialist for at least part of this final stretch as they try to hold off the Twins in the American League Central. With Nate Robertson now in the rotation, Seay and rookie Fu-Te Ni were the only lefties in the Detroit bullpen.

Seay, in his third season as Detroit's primary lefty reliever, was a major reason behind the surprising efficiency of the team's bullpen. Though he had a 3.88 ERA over 66 appearances, his 28 holds lead the Tigers, and he's been ranked among the best in the AL for much of the season. Left-handed hitters are batting .255 (28-for-110) against him, while right-handers are hitting .229.

To improve its depth, Detroit added fellow southpaw Clay Rapada to the active roster. The sidearmer was already on the 40-man roster but was not among the Tigers' September callups when rosters expanded. He posted a 4-2 record and 2.76 ERA with 47 strikeouts over 45 2/3 innings for Triple-A Toledo.

Between Seay's lefty-on-lefty matchups and his setup work, it's too varied of a role for one of Detroit's younger relievers to fill on his own. Between Ni, prospect Rapada and former top Draft pick Ryan Perry, the Tigers are going to try to do their best.

The fact that Ni has a chance to step up against some of the tougher left-handed hitters is a direct benefit of having two lefties in the bullpen. Leyland has largely managed to pick and choose his spots for Ni since the Tigers called him up at the end of June.

That said, Ni has had a share of key lefty-against-lefty matchups already. Grady Sizemore went 0-for-5 off him this year. Travis Hafner went 0-for-2, as have Justin Morneau and Carlos Pena. Joe Mauer, by contrast, has two walks and a sacrifice fly without an official at-bat, while Shin-Soo Choo had two hits and two RBIs in as many at-bats.

Those numbers against Mauer and Morneau could be particularly important if Seay isn't ready to return Monday, when the Twins and Tigers begin their four-game series at Comerica Park that will help decide the AL Central winner.

Add up the numbers, and left-handed hitters are just 7-for-58 against Ni with seven walks and 12 strikeouts. He did his job Friday by retiring White Sox catcher AJ Pierzynski in the eighth inning.

"I think he's adjusted pretty well," Leyland said. "All of a sudden, his responsibilities are going to be a little bit different here. There's no telling how that's going to work out."

Perry doesn't throw left-handed, obviously, but Zach Miner's relative success against left-handed hitters could earn Perry some more opportunities in the middle innings as a trickle-down effect.

More pressing for Perry coming into the weekend was the Tigers' need to bridge the gap between starters Eddie Bonine and Nate Robertson and the late end of the Tigers bullpen.

"I think he's going to be a big key in this series," Leyland said. "Perry's going to need to be ready, and he's going to need to be effective."