The Cubs have placed Jon Lester on the 10-day IL with a hamstring injury suffered running the bases after his RBI double on Monday. The team performed an MRI today, but the results, and the severity of the injury have not been reported at this time.
In the short run, the Cubs will need to replace Lester in the rotation for Sunday's game. With Mike Montgomery on the IL, the most logical choice as a fill-in is Tyler Chatwood, who was stretched out in spring training, and has proceeded to throw at least one inning in each of his relief appearances since the beginning of the season.
If they choose to dip into the Minors for a replacement, Duane Underwood, Jr. is the only fully healthy and stretched out option down in AAA. Both Alec Mills and Adbert Alzolay were left behind in EXST. Justin Steele was assigned to AA Tennessee, but he is on a limited pitch count as they slowly stretch him out after a minor injury in spring training. I'm not sure the current status on Mills, but the front office reported Alzolay would not be available until early May.
Should they go outside the 40-man (which I doubt given Chatwood's availability) for a fill-in starter or piggyback, Colin Rea offers the most experience (25 career MLB starts with SD). Duncan Robinson is lined up in the Iowa rotation and fully stretched out as well.
Kyle Ryan, Randy Rosario, and Allen Webster have previous starting experience and are capable of working 2+ innings should they choose to go with a piggyback or bullpen game as well.