During a Feb. 4 news conference at Yankee Stadium, Pettitte said that he felt physically ready to begin pitching, but that he did not feel like his heart was where it needed to be.
The left-hander also said, "I will not pitch this season, I can assure you of that, and I do not plan on pitching again."
Posada said that he spoke with his longtime teammate around the time of the decision, and Pettitte was having mixed emotions.
"He said he was running and throwing and everything felt great," Posada said. "He was looking forward to coming to Spring Training, and then he started looking around at the faces of his kids and it was tough to leave, I guess."
Yankees manager Joe Girardi is hopeful that Pettitte will show up somewhere this season to visit with his former teammates, but he isn't exactly banking on the idea that Pettitte would come back to pitch.
"Andy has always been pretty much true to what he says, from the guy that I've known, a man of his word," Girardi said. "And he said he wasn't coming back.
"He didn't put out the possibility that he might come back the following year, but he said multiple times, I don't plan on playing this year. But there are only a few things in life that are guaranteed."
With Pettitte's retirement, the "Core Four" of Posada, Pettitte, Derek Jeter and Mariano Rivera has dwindled by one. Posada isn't sure what catchy nickname the remaining three players should be given.
"I have no idea," he said. "The 'Core Four' sounded good."
The catcher rescinded his statement slightly a few hours later, however, possibly awakening to the realization of the situation.
"What I meant was that it would be nice to see him again," Posada told the New York Post later on Tuesday. "But he has made his decision."