White, 28, who last pitched on Aug. 28, has appeared in a career-high 52 games this season, compiling a 3-2 record and 2.80 ERA with one save. Though he's not overpowering, striking out 28 batters in 64 1/3 innings, he has held opposing hitters to a .216 batting average.
White, who was invited to Spring Training as a non-roster player, has been used primarily in middle relief, providing a solid bridge to right-handers Mark Lowe and David Aardsma.
His 52 appearances rank third on the staff, behind only Aardsma (64) and Lowe (63).
After being promoted from Triple-A Tacoma in mid-April, the former Rule 5 Draft selection out of the Braves organization reeled off four consecutive scoreless outings the remainder of the month and performed well in May. Three of the five earned runs he surrendered during 19 1/3 innings that month came in one game -- May 9 against the Twins.
Otherwise, he was superb. He put together a 17-inning scoreless streak that ended on June 13 against the Rockies.
But White began feeling discomfort in his right shoulder during the first stop of a three-city road trip against the Dodgers, Yankees and Red Sox in late June.
"We sat him down for a few days and he came back from it," Wakamatsu said.
Fingers are now crossed that the MRI will show nothing is seriously wrong and a few days of rest is all that's needed.
Losing White for the remainder of the season would remove a key piece of the bullpen that has been solid all season. Wakamatsu said right-hander Carlos Silva, currently on a rehab assignment with Triple-A Tacoma, would be used out of the bullpen when he returns.
Meanwhile, the anticipated return of right-handed starter Brandon Morrow from the Tacoma club has been put on hold. It had been reported that Morrow would start against the Rangers this weekend, but Wakamatsu said on Tuesday that Morrow will start Friday night's playoff game for the Rainiers.
"We talked about what is best and for now, the fact he had a little blister and some tightness in his [right] forearm the last time out, we figure the best thing is for him to stay down there," Wakamatsu said.