"I don't think you work that hard -- and take baseballs home and try to play catch without someone seeing you -- if you're not planning on trying to come back," Girardi said.
"It might have been a situation where he was trying to see how he felt, to see if he thought he could come back. I don't think any player ever wants to go out that way."
Rivera, who turns 43 next month, called general manager Brian Cashman last week and confirmed that he intends to pitch next season. Rivera is currently a free agent, and Cashman said he will work with agent Fernando Cuza to hammer out an agreement.
Girardi said that he believes he can count on Rivera to return to the closer's role next season, coming off the career-threatening torn right anterior cruciate ligament he suffered in May.
"My thought process is, when he left, he was throwing the ball well," Girardi said. "I know he's another year older, but he's been able to fight the time clock a little bit. I expect him to be productive."