OAKLAND -- Brandon Moss, still sidelined by a sore right ankle, had an MRI exam Sunday morning to determine the extent of the damage.
"It's gotten to the point where we're a little concerned," manager Bob Melvin said. "We want to get to the point where we can be precise."
Moss was injured while getting out of the batter's box Friday. He said at the time he didn't think the injury was serious and that he might miss a day or two, and Sunday was the second game for Oakland without the first baseman/outfielder in the lineup.
The slugger has been the A's cleanup hitter for most of the past six weeks and owns a .272 batting average with 19 homers and 62 RBIs. The RBI total is tied with third baseman Josh Donaldson for the team lead and is tied for sixth-best in the American League.
"Seeing him at the tarp gave me some trepidation," Melvin said.
The manager has been trying to be protective of the probable American League starter at third base because of Donaldson's back issues. He has given Donaldson two days off and had him DH twice.
As a result the A's won't have to make a roster move with him until Monday.
The addition of Hammel could force left-handed starter Brad Mills (1-1, 4.41 in three starts) off the roster.
Hernandez is 2-0 in three starts with a 2.49 ERA against the A's this year and 17-7 with a 2.64 ERA for his career against Oakland.
"It just made sense," McClendon said of having his best pitcher going against the A's. Hernandez is 7-1 with a 1.72 ERA in his past 11 starts.
"It's not too bad," Vogt said. ``I'll be OK out there."