The spot would have gone to Scott Proctor, but manager Joe Torre said he was not comfortable warming up Proctor repeatedly because he missed two months of the season with elbow tendinitis. Torre on Tuesday also said Troncoso would probably be the replacement, but the staff was split.
"I jumped the gun," Torre said Wednesday. "It wasn't an easy decision. McDonald brings a little more to the table. We're comfortable with what we've seen. We feel he can handle it emotionally."
McDonald impressed the staff with his calm demeanor, as well as his effectiveness against left-handed hitters (2-for-17, .118) and more varied pitch repertoire. Troncoso is a sinkerballer that was equally effective against righties and lefties.
Troncoso was told over the weekend he would not make the club and would stay in shape in the Arizona instructional league, but at the last minute was told to pack for Chicago. McDonald was in the mix since the weekend, was told before Tuesday's practice that he probably wouldn't be included on the roster, then was told Tuesday night that he was.
Only days ago, the 23-year-old McDonald was such a long shot he didn't even earn a page in the club's postseason media guide, instead being grouped with other players also not expected to participate in the postseason.
After being slowed by a sore arm most of the spring, McDonald split time between Double-A Jacksonville and Triple-A Las Vegas and was named the franchise's Minor League pitcher of the year with a 7-4 composite mark and 3.26 ERA.
He received a Sept. 1 callup to the Major Leagues, working only six innings in four appearances for the Dodgers, but allowing no runs and a .227 opposing batting average. The Long Beach native was an 11th round Draft pick in 2002 that spent one year in the Minors playing the outfield to rest a tired pitching arm.
"I'm still in shock a little bit. When they told me I was making the trip, I couldn't talk for a while. I was speechless," McDonald said Tuesday, when told there was a chance he would be activated.
"I've always been a starter, and being a reliever you go in with a little different mental approach. As a starter, I might set up a hitter for later in the game by showing a pitch. As a reliever, you got to go right after them."
McDonald said he wasn't awed by the pressure of the playoffs and Wrigley Field.
"I believe in myself," he said. "I expect to get the job done. I have confidence that they have confidence in me."
Troncoso had two stints with the Dodgers, including the final 3 1/2 months of the season. He pitched in 32 games with a 4.26 ERA.
Ken Gurnick is a reporter for MLB.com. This story was not subject to the approval of Major League Baseball or its clubs.