Votto, who will miss three to four weeks, had the surgery on Tuesday and was placed on the 15-day disabled list. The Reds recalled right-hander Todd Redmond, who was acquired from the Braves on Saturday for infielder Paul Janish, from Triple-A Louisville.
"Very tough news on Joey," Reds manager Dusty Baker said after Monday's 5-3 loss to Arizona. "We've got time to get him fixed, and hopefully other guys pick him up in his absence. Joey is in great shape and heals very quickly. We knew it was something, especially with the fact he wasn't healing. We just have to tighten our belts and everyone else has a job to do."
Votto originally injured his knee sliding into third base during a June 29 game in San Francisco and missed two games. He was 9-for-32 (.281) since he returned on July 3 and also went 0-for-3 while starting last week's All-Star Game in Kansas City. While he was 2-for-3 in Sunday's 4-2 Reds win over the Cardinals, he was a combined 0-for-8 in the first two games of the series.
The 28-year-old Votto underwent a MRI exam Monday, which confirmed the tear.
"He felt OK the next couple of days, then it got sore," Reds general manager Walt Jocketty said. "Back and forth, he felt he was OK. Then the last couple of days, it started bothering him some more."
Votto was examined during the team's West Coast trip but didn't have the MRI until Monday.
"He didn't request it until then," said Jocketty, who expected the surgery to be a 20-minute, minor procedure. "He said it wasn't a problem until the last couple of days. We decided to wait to have it done today."
Votto, who signed a 10-year, $225 million contract extension in April to keep him with Cincinnati through 2023, is batting .342 with 14 home runs and 49 RBIs and a league-best .465 on-base percentage. He has appeared in 86 of Cincinnati's previous 88 games and missed only three starts.
Jocketty acknowledged that Votto could have kept playing with the tear, but that it didn't make sense in the big picture. Votto confirmed that sentiment via a statement issued by the team.
"It is in my best interest and in the best interest of the team to do it now, so that I can be healthy during the last two months of the pennant race," Votto said.
The Reds, who entered the night winners of six straight games and had a one-game lead over the Pirates for first place in the National League Central, haven't been super sound offensively even with Votto's presence. They came into the day ranked 10th in the NL with a .249 team average and much of the lineup, with the exception of Votto, has struggled to drive in runners in scoring position.
"I saw him in the parking lot when I was driving in and he looked a little funny," pitcher Bronson Arroyo said. "I said, 'You playing today?' He said, 'No, I need to get surgery.' It's terrible news for us, obviously. As it is, he's the cornerstone of our lineup and right now, it's the time of year where it feels like we're going to have to press and keep pressure on the teams in this division in order to stay on top."
Rookie Todd Frazier started in Votto's place on Monday against the D-backs and went 1-for-4. Frazier filled in well for Scott Rolen over extended periods this season already. On days that Frazier needs to replace Rolen, veteran Miguel Cairo can also play first base.
"We'll see what we can do internally, initially, to try and replace him," Jocketty said. "It's going to be difficult to replace Joey Votto, although I think Frazier can give it the best shot."
To take Votto's roster spot, the Reds could summon veteran switch-hitter Dioner Navarro. Although not on the 40-man roster, Navarro entered the day batting .339 at Triple-A Louisville. Xavier Paul, who was signed to a Minor League deal over the weekend, began Monday batting .323 and hits left-handed.
But the hottest-hitting option at Louisville could be recent arrival Henry Rodriguez, who is on the 40-man roster. Rodriguez, a 22-year-old who plays second base and third base and is a switch-hitter, was also batting .339 this season but has only nine games logged at the Triple-A level.
"I think we've got the guys here to do it, the personnel to step up," Reds right fielder Jay Bruce said. "In the scheme of things, [Votto is] going to be gone for a little bit, but he'll be right back. I'm confident in the guys we have on the team. Obviously, when you lose someone of Joey's stature in the lineup, it's a lot different. He's pretty much the only one like Joey. We don't have a choice."
With the July 31 non-waiver Trade Deadline approaching, Jocketty felt no extra urgency to replace Votto's bat specifically via a deal.
"It doesn't affect it any differently," Jocketty said. "We're still going to look for the same things. Joey will be back. So we'll just keep working on what we've done to this point."
No matter how you slice it, no replacement can make up for the loss of Votto, who won the 2010 National League Most Valuable Player Award. He entered the day leading the Majors with 36 doubles, the NL with 66 walks and was in the top 10 in almost every statistical category this season. He also was a team-best .371 hitter with runners in scoring position.
The Reds will now face their biggest test of the season of trying to get by without Votto.
"I guess the good part of it is if you can weather the storm over the next two or three weeks and still be in the thick of things when he gets back, it gives us a lift when he does come back," Arroyo said. "For now, it's not a good prognosis for our team."