CHICAGO -- Brandon Inge had spent the better part of a month wondering what was the cause of his body aches, headache and uncharacteristically low energy. He has his answer, and Detroit has a hole to fill at third base.

The Tigers placed Inge on the 15-day disabled list on Friday with mononucleosis, a relatively rare condition for a baseball player. The club recalled catcher Omir Santos from Triple-A Toledo to take his place for the moment.

Santos, who hasn't played since May 26 because of an injury, most likely will have a short stay with the club until Magglio Ordonez is ready to return from the disabled list. Ordonez began a rehab assignment on Friday with the Mud Hens, and could rejoin the Tigers as soon as Monday for the start of their three-game series at Texas.

How much time Inge will miss is anyone's guess. Tigers athletic trainer Kevin Rand declined to comment to reporters, citing privacy laws since it wasn't a baseball-related injury. Inge did not respond to a text message on Friday. Manager Jim Leyland said he doesn't know any timetable.

"It is what it is," Leyland said. "A guy goes down, you play somebody else."

History isn't much more of a guide. Reds pitcher Bronson Arroyo was diagnosed this spring and didn't miss a start, but has had a noticeable downturn in his stats. Red Sox shortstop Jed Lowrie missed the first half of last season with the condition. Then-Angels prospect Casey Kotchman struggled with it for much of the 2006 season. Hayden Simpson, the Cubs' first-round pick in last summer's First-Year Player Draft, was diagnosed in June and didn't resume baseball activities until earlier this year.

It appears, though, that Inge could've been struggling to play through it for a while. Inge had mentioned since early May that he was dealing with his symptoms, but said he was starting to feel better during the just-completed homestand. He underwent blood tests two weeks ago but had no immediate answer on what was bothering him.

"I don't think he's been himself for a while," Leyland said, "but I don't know if that has anything to do with it or not."

At the risk of a bad pun, Inge had had an anemic season offensively, even for his up-and-down standards. He owns a .211 average (34-for-161), but more surprising, he's had just nine extra-base hits, one of them a home run. He has 44 strikeouts in 181 plate appearances.

Just two of those extra-base hits came in May, even though he hit for a slightly higher average (.219) than he did in April (.212).

The news caught his teammates by surprise. They knew about Inge's symptoms but never figured on this diagnosis.

"I came in here thinking he was going to be here, and he wasn't. It was definitely a shock to me," said catcher Alex Avila, whose locker sits next to Inge in Detroit and usually on the road. "But with him, any type of injury he has, you can't tell, because he doesn't show it. He probably felt like crap on the inside. A lot of people I've known who have had mono, it was hard for them just to get out of bed."

The Tigers have four infielders capable of filling in for Inge at third. Two of them will handle the bulk of the playing time, according to Leyland. Utility man Don Kelly, who has spelled Inge at third on his days off his season, is expected to handle most of the starts against right-handed pitchers. Danny Worth, who played the hot corner for Toledo before he was called up a week ago following the Scott Sizemore trade, will start against lefties.

Ramon Santiago and Ryan Raburn are also capable of playing there, though they have split most of the past week at second base since that spot opened up.