HOUSTON -- Taiwanese pitcher Fu-Te Ni is getting his shot in the big leagues. Nate Robertson, meanwhile, is facing the likelihood of surgery, albeit a relatively minor one.

The Tigers placed Robertson on the 15-day disabled list Sunday morning with a mass in the inner portion of his left elbow. They replaced him with Ni, whose contract was purchased from Triple-A Toledo. Ni arrived in the Tigers' clubhouse at Minute Maid Park late Sunday morning along with his translator and was available to pitch.

The move provides the side effect of a shift in Detroit's bullpen, where Robertson's struggles had limited his appearances recently. The veteran made the club as a member of the 'pen after spending the better part of five years in the rotation. He had a solid first few weeks as a long and middle reliever, going 5 2/3 innings with two runs allowed in his first five appearances, but his struggles had grown more pronounced lately.

Robertson owns a 1-0 record and a 7.71 ERA, allowing 18 runs on 25 hits over 21 innings with 14 walks and 17 strikeouts. Left-handed batters were hitting .326 against him with an .873 OPS this season. He had retired just two of nine batters faced over his past four appearances, and neither of the lone batters he was brought in to face Wednesday against the Cubs and Friday against the Astros.

The mass of tissue in his elbow likely played a part in that. Head athletic trainer Kevin Rand compared it to a cyst, and said Robertson had complained of numbness and tingling in his left ring and pinky fingers, meaning the mass was pressing on his ulnar nerve.

"He's had the masses, but it's like someone who has bone chips," Rand said. "People who have bone chips or bone spurs in their elbow, they don't do anything with them until they become an issue. It's something that had not affected him in any way pitching prior to this. It's starting to affect him with numbness and tingling in his hand."

Robertson returned to Detroit on Sunday for a Monday exam by team doctor Stephen Lemos, who's expected to perform surgery to remove the mass Tuesday. How that procedure goes will determine what sort of timetable Robertson faces to get back to pitching. Because Robertson had Tommy John ligament replacement surgery in the same area in 1998, they're hoping they can go in his elbow and minimize the scar tissue.

Ni's arrival makes him the first Taiwanese player to pitch for the Tigers and the sixth Taiwan-born player to make the Major Leagues. The Tigers signed the sidearming lefty to a Minor League contract this past offseason out of the Chinese Professional Baseball League and invited him to Spring Training before putting him in the bullpen at Triple-A Toledo.

Ni was arguably the most effective reliever for the Mud Hens over the course of the season so far, going 3-0 with a 2.60 ERA in 24 appearances. He allowed 31 hits over 34 2/3 innings with nine walks and 32 strikeouts. Left-handed batters hit just .184 with one home run against him, thanks in part to an improved breaking ball, according to bullpen coach Jeff Jones.

"Give him a shot. See what he looks like," manager Jim Leyland said. "We'll use him to get some lefties out."