Downs pitched in Houston's 10-5 loss to Seattle on Friday at Minute Maid Park. He allowed a home run to Dustin Ackley in the eighth inning before leaving with discomfort in his right oblique.
"Downs goes out [Friday] night and aggravated his oblique again," said Houston interim manager Tom Lawless. "He's going to get treatment and we'll see what happens. Obliques are strange. Twisting and everything. Pitchers twist all over the place. That may be it for him."
The right-handed Fields hasn't pitched since Sept. 9 when he pitched a perfect ninth inning at Seattle.
"He tried to go the other day and it didn't work out very well," said Lawless. "We'll see how he goes."
Richard Dean is a contributor to MLB.com. This story was not subject to the approval of Major League Baseball or its clubs.