Sale threw in the outfield for 10-15 minutes before Game 4 in Houston, but manager Alex Cora said he didn't throw a bullpen session as planned. David Price will start Thursday instead.
Cora said the lanky lefty ace was still feeling weak and had lost some weight because of a stomach illness that forced him to spend a night in a hospital after he started the ALCS opener against the Astros.
"He's feeling better compared to yesterday, but physically he's not there yet," Cora said before Game 4. "So I think if necessary he'll pitch Game 6. He feels that he'll be ready for that one. And we'll go from there."
The Red Sox got an 8-6 win in Game 4 on Wednesday night to take a 3-1 lead in the best-of-seven series. Game 6, if necessary, would be Saturday night in Boston.
Dave Dombrowski, the team's president of baseball operations, said Sale told him that "he's fine and he'll be ready to go on Saturday."
Corda said there's "no chance" Sale will pitch in any situation in Game 5.
Sale did not speak to reporters Wednesday.
Cora said earlier this week that Sale started feeling ill right after Game 1 and went to a hospital early Sunday morning.
Sale was released Monday from Massachusetts General Hospital after remaining there overnight for observation. He rejoined the team Tuesday in Houston before Game 3.
The club hasn't specified Sale's ailment or treatment, though Cora has said it was nothing serious.
Sale struggled with his control in Boston's 7-2 home loss to the Astros on Saturday night. He allowed two runs, four walks and hit a batter in four innings.
The lefty made just one start between July 27 and Sept. 11 because of mild left shoulder inflammation, then made four starts during the rest of the regular season. He beat the Yankees in the Division Series opener and pitched one inning of relief in Game 4 as the Red Sox won the best-of-five series.