In a wider conversation about Rivera's career Tuesday evening, Girardi was asked how many more seasons he thought his former teammate could continue to close at a high level.
"I don't see any reason why he couldn't do it next year, I don't," Girardi said. "He's made it pretty clear that he doesn't want to [return], but I always say, you know, January rolls around and sometimes you have a different feel about what you want to do."
Rivera, 43, announced during spring training that his 19th season with the Yankees would be his last.
"I told you guys already. I don't know why we're talking about this. I've already made my decision [to retire at the end of the season]," Rivera said Tuesday night after earning his 40th save of the season by pitching a 1-2-3 ninth inning in the Yankees' dramatic 6-4 victory over the White Sox.
Rivera now has collected 40 or more saves in nine seasons, tying the all-time record held by Trevor Hoffman.
"I'm sure I'll talk to him at some point in the offseason," Girardi said, "and ... I'll tell him when the season's over, 'Take a month. Take a month and a half, two months, and make sure this is really what you want to do. Because once you do go, it's hard to come back.'"
Of course, Rivera wouldn't be the first dominant Yankees pitcher of his era to return to the big leagues after announcing he was done; Roger Clemens and Andy Pettitte made their own comebacks, with Clemens all but halting the 2003 World Series at the end of his farewell tour before signing with the Houston Astros three months later.
"I think he could still do the job, but knowing how hard it is to take your uniform off, you don't ever want to think, in my mind as a player, and I'm not speaking for Mo, but I never wanted to think, could I have played a little bit more. It was really evident for me because I physically couldn't stay healthy. But Mo has been pretty healthy this year," Girardi said in the postgame news conference.
Rivera has said his goodbye by meeting with longtime employees and fans of opposing teams to thank them for supporting the game. Told it would be difficult for Rivera to resume his career after staging his selfless victory tour of ballparks, Girardi said: "Well, I would tell him there's probably three National League cities we're going to next year that we didn't go to this year."
At his retirement news conference in March, Rivera ruled out a future change of heart, saying, "I want to stay home, close the door and do what's next."
To date, Rivera has offered no sign that he's eased up on that stance. The Yankees plan to honor him before their home game with the San Francisco Giants on Sept. 22.
"I believe he's going to retire," Girardi said. "But as I've said, sometimes when you're a player in the midst of a season and you're grinding it out, your mind is one thing, but when you get away for a couple of months and your body feels pretty good, your mind is another thing."
Information from ESPNNewYork.com contributor Mike Mazzeo and ESPNNewYork.com's Wallace Matthews was used in this report.