Those slots likely won't receive replacements until Monday and Tuesday, when the team's Nos. 4 and 5 starters are slated to take their turn in Anaheim.
In the meantime, the A's called upon three relievers to help a depleted bullpen that was forced to compile 8 2/3 innings of work following Ross' premature departure in an 11-1 loss to the Twins on Thursday. Right-handers Joey Devine and Fautino De Los Santos, along with lefty Jerry Blevins, were all available for Friday night's series opener in San Francisco.
Ross and McCarthy are without a return timetable but realize they're likely going to be sidelined for at least a handful of weeks, as both have to take proper time away from any activity before even thinking about throwing again.
While Ross' injury surfaced just Thursday, McCarthy admitted Friday that he's "been dealing with weird issues over the last two weeks."
"I didn't really feel this coming on until a few days before my last start," he said. "It just kind of turned into something where I couldn't recover."
McCarthy is rather familiar with the shoulder injury -- a stress reaction in his right scapula -- having incurred it three times before in his career. The 27-year-old righty, who signed as a free agent this past offseason, is hoping he caught it soon enough to avoid a prolonged rehab process.
"I've gotten hypersensitive to it, so when it came up last week I alerted them so that everyone was aware," he said. "I've gotten to the point where I can feel it right away. Coming out of my last start, we had it checked out and found it again.
"In the past, I've pitched with it for a while and usually I've stressed it way more than I've needed to and that leads to a longer period of rest, which usually means I'm out for the season. This one, if I can miss three or four starts -- obviously that's optimistic -- but I'd be happy. The goal is to not miss the entire summer again."
McCarthy most recently pitched seven innings against the Twins on Wednesday, giving up three runs via nine hits with no walks and one strikeout. He had put together a 1-4 record in nine starts despite compiling a 3.39 ERA following an impressive spring showing -- one walk next to 20 strikeouts in 26 innings -- that earned him a rotation spot ahead of Ross and Josh Outman.
Ross, meanwhile, was filling in for the injured Dallas Braden, who will miss the rest of the season after undergoing shoulder surgery earlier in the week. The young righty was earning all sorts of praise for his efforts in Braden's absence, which included a 3-2 record and 2.50 ERA entering Thursday.
"It's just disappointing," Ross said. "You finally get on a good little roll there and things are coming together and the team is playing some good ball. It [stinks] that I'll be sidelined for a little while.
"I wasn't feeling too good this morning, so I knew it was something, but I was hoping it wasn't going to be this long. Now it's just going to be some time before I can get back out there."
McCarthy knows this frustration all too well but said he's trying to avoid the lengthy self-pity parties he's thrown himself in the past.
"Honestly, I've gone into this one a little more positively because I've been throwing well and know there's a spot to get back when I'm healthy again," he said. "In the past, when I haven't been contributing much going into it, it makes it tougher.
"The bad thing is you start thinking about what it does to the rotation, what it does to the team, you start projecting all sorts of stuff and asking how I can be a consistent Major League starting pitcher. You start playing little mind games with yourself, and I had to get over that really quickly."
Among the leading candidates to grab a rotation spot is Outman, who hasn't pitched in the big leagues since July 2009 after undergoing Tommy John surgery. The lefty was a potential fifth-starter option in camp this year but struggled with mechanics and command, which led to his demotion.
Since, he's posted a 4-1 record and 4.78 ERA in eight starts for Triple-A Sacramento. His command is still a work in progress, evidenced by the 27 walks he's issued in 37 2/3 innings, but he's also struck out 30.
Fellow southpaw Bobby Cramer, who began the season in the A's bullpen before being optioned to Triple-A in late April, would normally be considered a strong option as well, but he's been on the DL with a lower back strain and isn't due to be reinstated until Sunday.
Considering the No. 4 spot won't come up again until Monday, Cramer is technically still a possibility, but manager Bob Geren hinted the club would likely be more inclined to call upon a healthier option.
That could be good news for Guillermo Moscoso, who also has big league experience and is already on the club's 40-man roster. The right-hander has compiled a 3-2 record and 4.02 ERA in eight games -- seven of them starts -- for the River Cats.
"There are several guys we'll be looking at," Geren said. "It's a definite loss, but we'll look forward, not backwards, and carry on. The offense is starting to swing the bats a little bit better, so we just need them to pick up the slack if we lose anything in the pitching department."