Posada was injured while sliding during the sixth inning of Monday's 6-4 loss to the Red Sox, and he underwent an MRI examination on Tuesday in New York. Posada is expected to be sidelined for two to three weeks.
"It is disappointing, but I know I can come back," Posada said. "But it's tough. It's tough on me, because I worked really hard to be here. I worked really hard on everything on my body to be back, and it's just one of those things that you can't control. I'm just trying to stay positive, but it's not easy."
Posada was hitting .312 with five home runs and 20 RBIs in 23 games before his injury. He has been diagnosed with a Grade 2 strain, which means that between 10 and 50 percent of the muscle fibers were torn in the hamstring.
"Hamstrings can be tricky," Yankees manager Joe Girardi said. "I hope it's just the two weeks. They'll treat this every day, and we'll see how it progresses."
As he was last year, Jose Molina is expected to replace Posada as the Yankees' full-time catcher until Posada returns. Molina entered Tuesday batting .257 with one home run and five RBIs in 13 games this season.
"I was shocked about [losing] one of our best hitters on the team at this point," Molina said. "He was the guy driving in runs, and so clutch. It's sad to see a guy who's so valuable for the team go down. I know Jorgie is a strong guy, and he will come back soon and keep helping us to win the World Series. That's our goal as a team."
Girardi said that Posada expressed disappointment in being held out of action, especially after missing much of last season due to right shoulder surgery.
"I told him I do feel bad for him because he worked so hard to get to this point," Girardi said. "All the rehab he went through every day this winter and every day in Spring Training -- to hurt his leg and have to go on the DL, I guess if you look at the positive side of it, he didn't hurt his shoulder again."
While Monday's game was delayed for two hours and 17 minutes by rain, Posada said that the damp weather was not a factor in the injury. He was replaced in the eighth inning by Hideki Matsui, who was inserted as the designated hitter.
"I don't think so," Posada said. "I was pretty warm. I was getting on the bike in between at-bats, I was stretching and doing all the stuff that I could do as a DH. I don't know. It was cooler, but I don't think it was the weather. I don't know, it just happened."
When Posada was sidelined last season with a weak throwing shoulder that eventually required his first stint on the disabled list and then season-ending surgery on July 30, Molina stepped in as the Yankees' regular backstop.
Molina batted just .216 with three home runs -- including the final one hit at the old Yankee Stadium on Sept. 21 -- and 18 RBIs in 100 games, but he wore down late in the season and said he needed to change his workout regimen to anticipate extra duty.
"When you're a backup, you have to prepare yourself differently," Molina said. "I learned last year I was wrong. I should have prepared myself like an everyday catcher. That way, it's not a surprise. I've been doing that all spring and through the first month of the season. I'm still a backup, but in my mind, I think that I'm starting every day."
In a corresponding roster move, the Yankees recalled catcher Francisco Cervelli from Double-A Trenton to replace Posada on the 25-man roster.
"I felt healthy, and now I've got this opportunity," Cervelli said. "It's sad for Jorge, because nobody wants these things to happen, but I'm here to try to help."
Best regarded as a defensive catching prospect, Cervelli was hitting .190 with two home runs and seven RBIs in 16 games with the Thunder, and was up briefly with the Yankees at the end of last season. He also played for Team Italia in the World Baseball Classic.
"I saw these guys on TV like 10 years ago," Cervelli said. "Now, I'm with them. It's a special moment."
Bryan Hoch is a reporter for MLB.com. This story was not subject to the approval of Major League Baseball or its clubs.