In the bottom half of the inning, Horst got his first big league at-bat and singled through the right side, sending home Jonny Gomes from second to temporarily give the Reds a one-run lead. The hit matched his career total in the Minors, where he went 1-for-11.
The lead didn't last long, as McCann launched his second homer of the night on a hanging slider to begin the fifth, tying the score at 6. But Horst settled down after that, striking out Eric Hinske and Freddie Freeman and getting Dan Uggla to fly out.
"I basically was going pitch for pitch and trying not to let the game speed things up," Horst said. "[I] just tried to slow down and stay in control."
Horst exited with two outs in the next inning, as he surrendered a one-out double to pinch-hitter Wilkin Ramirez before getting Schafer to fly out.
"He did a great job. He had a number of firsts," manager Dusty Baker said. "First strikeout, first hit, first home run allowed. A lot of firsts tonight. We're proud of the job he did."
Horst was the second Louisville pitcher to join the Reds in the last two days. Right-hander Mike Leake was called up on Friday to start against the Braves and took the roster spot of reliever Daryl Thompson, who was optioned to Double-A Carolina.
"We liked what we saw in Spring Training," Baker said before the game. "He threw strikes. He moved the ball around."
Horst gives the Reds two left-handed relievers, with Bill Bray being the other. The rookie posted a 1.73 ERA in 26 innings with 23 strikeouts in 12 relief appearances for Louisville.
"I got another lefty, an important lefty," Baker said. "He's pitching pretty good down there, too, they tell me. He was throwing multiple innings. So he's capable of going multiple innings, and I can save Bray for a more important time in a situation late."
Horst became the second Red to make his Major League debut this week. Todd Frazier pinch-hit in Monday's loss to the Phillies.