MILWAUKEE -- Rich Hill's tentative return to the A's rotation this weekend has been delayed. The A's left-hander aggravated his left groin injury during a bullpen session in Milwaukee on Wednesday, and he will return to the Bay Area on Thursday for an MRI.
Hill was penciled in for a start in Cincinnati on Saturday, but there's a "definite possibility" he'll now go on the disabled list, A's manager Bob Melvin said.
The American League Pitcher of the Month for May was supposed to throw 35 pitches on the sidelines at Miller Park on Wednesday, but Hill said he felt a "grabbing, pulling sensation" on his 33rd and 34th pitches, "essentially what I felt in the seventh inning [10 days] ago."

That's when Hill last appeared in a game for the A's, exiting with a 2.25 ERA, second best in the AL.
"If that's happening in a bullpen session with the intensity we're at, when you factor in competition and adrenaline and all that, we'll see what the MRI says," Hill said.
Sonny Gray, Kendall Graveman and Sean Manaea will pitch in Cincinnati this weekend, with Jesse Hahn getting the nod for Monday's home game against the Rangers. That leaves the A's in need of a starter for Tuesday.
Worth noting
• Right-hander Henderson Alvarez (shoulder) will make a rehab appearance with Class A Advanced Stockton on Monday. He's scheduled for two to three innings.
• Relievers R.J. Alvarez and Liam Hendriks are on track to rehab with Triple-A Nashville on Thursday and Friday, respectively.