From C. Trent Rosecrans at thelotd.com:

Aaron Harang will fly back to Cincinnati from Chicago to undergo an MRI following his career-high seven-walk performance on Tuesday night, the team announced on Wednesday.

Harang has struggled since throwing four innings in an extra-inning game in San Diego on May 25, going 1-5 with a 7.31 ERA since that outting.

Harang missed his scheduled start on Saturday and was pushed back to Tuesday with forearm stiffness. The team said he's still bothered by the injury.

If Harang is put on the disabled list, Daryl Thompson could step right in for him. Thompson allowed one run on six hits in six innings in Louisville on Tuesday, picking up the win for the Bats. Thompson, optioned to Louisville last week could be recalled if the Reds placed a player on the disabled list. Otherwise, Homer Bailey, who pitched well on Monday, could be recalled to start on Sunday.

I hope the injury is minor, and a couple weeks of DL time is all he needs.  Obviously the 5-inning relief appearance could be a reason, or it could be something that has nagged him most of the year.  He experienced a similar pain in 2006, and skipping a full turn remedied the problem.

I say no matter what, Harang should not make the start Sunday.   Give him the rest and hope it heals up for post all star break.

As Trent Mentioned, the Reds have several options.  If they choose not to DL Harang, they could hope Jeremy Affeldt can give them 4 or 5 innings and do a Bullpen game.  In addition to Homer or Darryl, Matt Maloney or Matt Belisle could be options. (Belisle seems unlikely)  Bobby Livingston is on his way back, but still looks to be atleast another 2 weeks off (unless there is a setback)