The rookie right-hander will make his fourth career start Tuesday night when the visiting Athletics continue their four-game series against the Los Angeles Angels.
With Bartolo Colon in the midst of a 50-game suspension and McCarthy recovering from an epidural hemorrhage, brain contusion and skull fracture after being hit in the head with a line drive Wednesday, Oakland (80-60) finds itself using four starters with less than two full seasons of experience.
Straily (1-0, 3.18 ERA) was recalled from Triple-A Sacramento after McCarthy's injury. He started three games in August - all wins for the Athletics.
Straily pitched well in two of those, but struggled against the Angels on Aug. 8. He allowed five runs on four homers, including two to Kendrys Morales, over 4 2-3 innings of a 9-8 victory.
Despite Straily's lack of experience, Oakland remains confident the 23-year-old can pitch well.
"I think he's been through all the firsts and knows what it's all about here," manager Bob Melvin told the Athletics' official website. "Again, we're not afraid to throw some young pitchers out there. We got a lot of young starters in our rotation right now and he will just add to that mix."
One of those, Jarrod Parker, allowed one run and three hits over seven innings of a 3-1 victory Monday to snap the Angels' six-game winning streak, which began with a sweep in Oakland from Sept. 3-5.
"It's fun," Parker said. "These are the kind of starts that we need, and everybody is going to feed off this kind of energy."
Oakland has allowed two or fewer runs in four straight games after giving up 21 in three games to the Angels.
The A's have won 19 of 24 and have a two-game lead atop the AL wild-card standings while pulling within three games of idle Texas for first in the AL West.
Meanwhile, the Angels (77-64) fell 1 1/2 back of Baltimore for the second wild-card spot.
After sweeping three games from Detroit over the weekend, they were held to four hits Monday. One of those came off the bat of Mike Trout, who is hitting .328 this season but .245 (12 for 49) in 12 games against Oakland.
"I'm not surprised anymore at what they're doing," Torii Hunter said of the A's. "I mean, we were surprised when they first started doing it in early August, but I'm not surprised any more. They're playing the game, and they've got some really good arms over there."
Williams' last appearance was against Seattle on Sept. 2, but he hasn't started since allowing five runs in six innings of a 5-1 loss at Detroit on July 19. He's 0-5 with a 7.53 ERA in his last five starts.
"Jerome's been pitching well when he has the ball," manager Mike Scioscia said. "So, we all feel he's going to go out there and give us a chance to win."
Williams is 0-1 with a 3.68 ERA in two appearances - one start - against the Athletics this year. On May 21, he went 6 1-3 innings, giving up two runs and walking five in a 2-1 loss.