The Yankees will be using a six-man rotation, something they have done at various times this season.
When Sabathia, 35, went on the disabled list with an inflamed right knee on Aug. 24, Girardi thought the lefty might not pitch again this season. Now it is likely Sabathia will face the Baltimore Orioles on Wednesday.
"I am surprised," Girardi said.
In a simulated game at Yankee Stadium on Friday, Sabathia threw 60 pitches in four innings. There were no fielders and thus no line from the game.
"Now we just have to see how the knee reacts to what we did today," Girardi said. "If it is good, then we'll probably put him in the rotation."
Sabathia missed most of last year because of knee surgery. Before that procedure, there was some thought it could end his career.
Upon his return this spring, Sabathia chose to wear an athletic sleeve on the knee instead of a brace. During the simulated game, Sabathia wore the brace, which he said was a big improvement.
"I feel great after this," Sabathia said following the simulated game. "I'm excited."
Sabahia is 4-9 with a 5.27 ERA. He is owed $25 million for 2016 and, if he doesn't hurt his shoulder, another $25 million for 2017.
The Yankees have used a sixth starter at various times during the year to protect Masahiro Tanaka and their starters.
"It helps our rotation all around," Girardi said. "It also helps to have a left-hander in your rotation. Physically we have had concerns all year and we have kind of done this all year."