It depends a lot on how soon manager Jim Leyland means by "real soon" when he talked Saturday night about eventually getting Fister back.
From a pure pitch count standpoint, Fister has already been stretching out his arm. He threw 50 pitches in his first bullpen session Thursday, then topped that in his latest mound session Saturday. One would expect the Tigers would give him close to a normal pitch count Wednesday night.
From there, the question as to one rehab start or two comes down to how much time the Tigers want Fister to get against live hitters. He hasn't seen live hitters in three weeks since straining a muscle near his left rib cage.
If the club decides one rehab start is enough, then it's feasible Duane Below's start Monday against the Royals could be his only one in place of Fister. The Tigers are off on Thursday, so they could use the off-day to push back Below's spot and move up Rick Porcello and possibly Justin Verlander to start on normal rest.
In that case, Porcello would face the White Sox on Sunday. If the Tigers pushed up Verlander to pitch Monday at Seattle, then they wouldn't need a fifth starter until next Tuesday at Seattle. Fister would have enough rest at that point to start, if that's what the Tigers wanted.