"He had some back spasms last night and was working hard to get them out of there," A's manager Bob Melvin said. "We wanted to at least wait until this morning to see how he felt, but we had a pretty good idea last night he wasn't going to start.
"We'll see how he feels in Baltimore, and we'll get him in there when we feel it's the proper time to get him in there, but at this point, not sure."
Melvin said after Thursday's loss that Gray is "a little better," but he firmly noted Gray is not an option for Friday's start. He also ruled out right-hander Chris Bassitt, who is scheduled for Saturday on five days' rest. That means the A's will be calling upon an arm from Triple-A Nashville to face the Orioles.
Left-hander Brad Mills, who is scheduled to start for the Sounds on Friday, is a strong option to get the nod. He's 4-11 with a 4.45 ERA in 22 starts for Nashville this season and has a 1.56 ERA in three career starts against the Orioles, his last coming in 2012 as a member of the Angels.