They're still looking.
"We can't figure it out," manager Jim Leyland said. "As with all the pitchers, he's been checked out, and there are no red flags medically, so that's good."
Porcello is pain-free, so the Tigers plan to send the sinkerballer to the mound again on Monday in Kansas City against the same Royals lineup that piled up hits in the air against him on Wednesday at Comerica Park.
Porcello's velocity within that outing fell from 92-94 mph early to 89-90 mph in his fourth and final inning, according to data from MLB.com Gameday.
"If he's [throwing] 89 and it's sinking like crazy, it's all right," Leyland said. "But if he's 89 and up, that's a no-no."
Porcello hasn't gotten through six innings in any of his last five outings. But then, neither have any of the pitchers on the staff who could replace him if he has to miss a start. Drew Smyly started in place of the injured Max Scherzer on Friday night and will be lined up to do the same Wednesday in Kansas City if Scherzer can't go.