McCarthy, who is 5-2 with a 4.92 ERA in 11 starts, came out of his most recent start against the Yankees feeling some soreness. An examination on Monday revealed the stress fracture in the shoulder blade.
"It's frustrating," Daniels said. "I feel for him."
Doug Mathis, who was 4-2 with a 2.89 in 10 starts for Triple-A Oklahoma, will start on Tuesday. This could mean that Derek Holland will stay in the rotation once Matt Harrison returns from the disabled list, but the Rangers want to take a look at Mathis, who was 3-0 with a 0.43 ERA in his last four starts at Oklahoma.
"We're going to let Doug pitch tomorrow and see how he does," Daniels said. "We're not ruling Doug out. He's deserving of the opportunity."
Daniels did not rule out looking for starting pitching outside the organization through a trade.
"I'm open to it," Daniels said. "It's something we'll look into, but it's easier said than done. My expectation is we'll go with the guys we have. If there is a quality guy available, we'll look into it, but it's easier said than done."
Harrison, who is sidelined with a strained left shoulder, threw in the bullpen on Monday. He is expected to come off the disabled list and start on Thursday. At that point, Holland was expected to go back to the bullpen.
That's an area that's been a concern for the Rangers for the past month, and it still is. Closer Frank Francisco remains day-to-day with a strained right shoulder. He's feeling better, but his status for Monday's game with the Blue Jays remains unknown.
Daniels has been talking with other clubs about bullpen help and said Monday, "That's still probably where my short-term focus is."
T.R. Sullivan is a reporter for MLB.com. This story was not subject to the approval of Major League Baseball or its clubs.