Lynn, a rookie who had not pitched since Aug. 9 because of a strained left oblique muscle, threw a scoreless sixth inning. He gave up a Jerry Hairston Jr. leadoff single and walked Prince Fielder intentionally with one out before inducing a Rickie Weeks double play.
The Cardinals left McClellan off the NL Division Series roster because he had suffered from "dead arm" -- a non-specific but painfully authentic pitching ailment. McClellan, who said at the time he thought he could pitch, gave up two hits and the Brewers' final run in the seventh inning.
Lynn, 24, has spent the past weeks preparing for games he didn't know he'd see this year. The Cardinals had to make a rally late in the regular season to win the NL Wild Card and had to triumph in a tight NLDS Game 5 over the Phillies, just so he could return and pitch in an opening game of the series that determines the NL World Series representative.
"It was fun," Lynn said. "I was working hard the last few months, just making sure that I was ready to go if need be. I'm happy to get that opportunity. I'm just trying to pitch as well as I can.
"I was ready to go whenever the phone rang down there, and I was able to get out of that inning with a double-play ball. That was big to keep it right there, give us a chance later on in the game."
While the Cardinals were winning tense games to push their way into the playoffs, Lynn was working simulated innings against September callups. There were no box scores or SportsCenter highlights, but Lynn knew they had to be approached the correct way.
"You've got to worry about day to day, to make sure you're not trying to push it too much and overdo it, or not tell the truth when something's bothering you," Lynn said. "I just had to be honest with myself and with the training staff when I was rehabbing."
McClellan, 27, said he experienced no heath issues in his return to the mound. He just put himself in a tough spot by running up a 3-2 count to seventh-inning leadoff man Yuniesky Betancourt, who doubled. Carlos Gomez sacrificed Betancourt to third, then Jonathan Lucroy hit a pitch McClellan liked for an RBI single. That was it for McClellan.
"I thought I had pretty good stuff today, but they put together some great at-bats," McClellan said. "They got me today. We get ready to go tomorrow.
"I felt pretty good, didn't feel like I was rusty. I felt like I had been throwing for awhile. I'll just look at film and see what I can do better."
The key was the Betancourt at-bat. Betancourt ran the at-bat to eight pitches, then took advantage of McClellan's aversion to walking leadoff hitters by jumping a sinker over the plate. Lucroy also hit the eighth pitch, a slider that actually was in a good location.
"I've always been a big believer that if you look at my numbers, I'd rather give up a hit than a walk," McClellan said. "Middle of the plate, up and he put a good swing on it. Obviously, I'd like to have not been in that situation."
Thomas Harding is a reporter for MLB.com. Read his blog, Hardball in the Rockies, and follow him on Twitter @harding_at_mlb. This story was not subject to the approval of Major League Baseball or its clubs.