Injuries and illness have limited City’s record goalscorer to just three starts in all competitions this season.
The 32-year-old Argentine, who missed the start of the campaign following knee surgery and then suffered a hamstring issue, tested positive for Covid-19 last week.
“The first three or four days he struggled,” Guardiola told reporters. “He felt so tired, but today he says he is feeling much better. Now is the time to test negative and when he can, come back here and see how he feels day by day.
“I cannot tell you when he will be back … But I am optimistic that he will give us, from February or March to the end of the season, some good months,” he said.
“Hopefully he can help us with the special quality he has. That’s why he is one of the best strikers ever – not just in England – of all time in football history.”
City, who have moved up to second on the back of six consecutive victories, face West Bromwich Albion later on Tuesday.