Instead, the trio will battle for the final spot in the rotation with the other two providing valuable depth during the season.
McCarthy was 8-6 with a 3.24 ERA in 18 starts for the A's in 2012.
"We were trying to target a guy that we thought was a [No.] 1, 2 or 3 [starter] and this guy was at the top of the rotation for a team last year for a team that won the AL West and had one of the best records in baseball," Towers said.
McCarthy's 2012 season came to a frightening halt Sept. 5 when he was struck in the head by a line drive off the bat of Erick Aybar. McCarthy suffered a skull fracture and a brain contusion and spent the rest of the season recuperating.
"He's somebody we've been watching for a long time. We tried to sign him two years ago before Oakland did," Towers said. "He's got good command of his fastball, which is paramount for us, especially in our ballpark, and it allowed us to add a veteran starter without having to move any of our young pitching depth."
Since signing with the A's, McCarthy changed his arm slot, throwing less over the top, which Towers said has given McCarthy better life on his pitches.
Over the past two seasons, McCarthy has walked just 49 while striking out 196.
Towers said adding McCarthy did not mean he was more likely to deal a young pitcher in trying to upgrade the shortstop position. During this week's Winter Meetings, there was a rumor that the D-backs and Indians were talking about a deal that would send Bauer to Cleveland in a package for shortstop Asdrubal Cabrera.
During the Meetings, Towers signed veteran free agents Eric Chavez, Eric Hinske and Wil Nieves to help bolster the bench, and the team selected right-hander Starlin Peralta in the Rule 5 Draft.
"If we had to start the season where we are right now, we're fine," Towers said. "If a deal is presented to us involving an outfielder or some of our pitching, we're in a better position where we've got depth now."
The D-backs would still like to move one of their outfielders. If Upton winds up staying, the team would likely look to move veteran Jason Kubel as a way to free up playing time for Gerardo Parra.
McCarthy's presence in the rotation does not mean that the D-backs won't add another pitcher when dealing an outfielder.
Tampa Bay's James Shields has been a name linked to a multi-team deal involving Upton.
"I'm just trying to make my club better," Towers said. "I can't tell you exactly if it's [adding] another starter or who knows. It may be nothing. We'll just have to wait and see."
Towers is certainly keeping busy while he waits.