The 6-foot-3 inch, 275-pounder has not pitched since June 29 when he appeared against the cross-town rival Cubs. He had experienced back soreness the previous day.

The White Sox have held him back because the discomfort continued.

"I don't want to take a chance in a situation like that," Manager Ozzie Guillen said Saturday on the team's Web site. "I don't think it's something to panic about, but it is a concern."

Guillen said he wanted to hear the results of the medical tests before assessing Jenks's immediate prognosis.

Jenks is 2-0 this season with an ERA of 1.95 and has converted 18 of 21 save opportunities, helping the White Sox to the top spot in the American League Central Division.