With veteran catchers Jason Varitek (knee) and Jarrod Saltalamacchia (collarbone) bruised and banged up, Red Sox manager Terry Francona needed to lean on the rookie Lavarnway to handle the duties behind the plate in the team's most important game to date.
Lavarnway delivered a major performance.
He hit two home runs, including a three-run shot, as Boston defeated the Baltimore Orioles, 8-7, at Camden Yards. The Red Sox remain dead even with the Tampa Bay Rays with the final game of the regular season for both clubs on Wednesday.
The kid with an Ivy League education from Yale knew it was a special moment.
"It definitely feels good. I feel like I can wear this jersey with pride, especially a little bit more now that I've helped out and I'm contributing," Lavarnway said. "That's what I wanted to do when I got here. I didn't want to just have a September call-up that was meaningless, so I was glad that I could help out today."
Prior to Tuesday's game, Lavarnway had played a total of 15 games with the Red Sox this season, his first in the big leagues, but it was his first start behind the plate. He started seven games as the club's designated hitter and played six games as a substitute catcher.
Francona informed Lavarnway on Monday night that he would be starting Tuesday, so the catching prospect did his homework. He studied the Orioles' hitters. He studied the Sox pitching staff too. His preparation was noticeable. His communication with the club's pitchers was evident.
Prior to the game, Lavarnway said he was confident he could do the job and it was obvious from his performance that he was not overwhelmed by the moment.
"He played his ass off," Francona said with a huge smile. "That was exciting. Besides what he did offensively, I thought he ran the game, I thought he had a lot of poise. That was one of the more exciting things to watch. We've seen a lot of interesting things over the years, but that was right near the top."
The Red Sox have struggled big time this month and Lavarnway gave the club a youthful spark similar to what Jacoby Ellsbury did in September 2007, a season Boston won it all.
"He carried himself so well and I'm sure his confidence level was growing as the game progressed," Francona said of Lavarnway. "He did a great job. Ellsbury's done it before and we've had guys do it before."
Lavarnway, who hit 32 home runs in Double-A and Triple-A combined this season, hit his first major-league home run in the top of the fourth inning, a three-run shot that gave Boston a 5-1 lead.
"I caught myself cheating a little bit on the 3-and-1 pitch and ended up checking my swing," he said. "So I just tried to stay short to the ball and stay within myself up the middle."
His solo shot to lead off the eighth, which made it 8-4, proved to be the winning run for Boston.
"It was awesome, man," Saltalamacchia said. "The kid comes up in a tough situation like that and he did so well. He handled the staff well, called timeouts when he needed to and slowed the game down. He made some big blocks in the last inning and a great play in that last inning."
The play Saltalamacchia was referring to came in the bottom of the ninth with the Sox clinging to a two-run lead. There was one out with runners on the corners for Baltimore when Matt Wieters, who had a two-run homer earlier in the game, stepped into the box against Red Sox closer Jonathan Papelbon.
Wieters hit a dribbler on the grass between the mound and the first-base line as Papelbon and Lavarnway both charged after it. Boston's rookie catcher scooped it up and got the out at first while the Orioles scored to make it a one-run game.
"The look [Papelbon] gave me was, 'You better field this ball,' so I went after it and tripped as I was throwing it, but I got the ball to first base," Lavarnway said.
Given the circumstances, it was a huge play.
"You've got to jump out of the box first and get to the ball," Saltalamacchia said. "And nine times out of 10 the pitcher makes that play, but he was smart enough to stay with it and make the play."
Lavarnway's two homers were a major contribution for Boston's offense, but the way he played defensively was equally important. He even threw out Adam Jones trying to steal third in the bottom of the second, and had numerous blocked balls.
OK, so now what?
Even though Lavarnway played so well, and Varitek and Saltalamacchia are all busted up, the chances of Lavarnway being in the lineup Wednesday are slim. After Tuesday's game, Francona did not say what his plans are, but Saltalamacchia said he's ready to go.
"I'll be ready to play tomorrow," Saltalamacchia said. "I'm good to go."
Saltalamacchia was removed from Monday's game in the eighth inning after a foul tip caught him directly on the collarbone. He was available to play Tuesday if needed, but Lavarnway had everything under control.
"It feels good but the job's not done," Lavarnway said. "We still need to go out tomorrow and win another ballgame."
Joe McDonald covers the Red Sox for ESPNBoston.com.