Gabriel Jesus has suffered a medial collateral ligament damage and is set for further tests to establish the extent of the injury, Manchester City confirmed on Monday night.
The Brazilian striker twisted his left knee in the 0-0 draw with Crystal Palace on Sunday and left the pitch in tears.
Manager Pep Guardiola initially hoped he would be out for "one to two months" but the club is not putting a timeframe on his recovery until he has been given a more thorough examination.
However, Gabriel wrote on Instagram that the injury is "not very bad" and will not required surgery.
"Today I went through an image exam on my left knee that diagnosed a small medial collateral ligament injury! Thank God it is not very bad and I won't have to go through any surgery procedure.
"Promise to be back as soon as possible! Thanks for all the fans support and positive thoughts! Happy new year! 2018 is going to be my year!"
The club said in its statement: "Manchester City can confirm Gabriel Jesus suffered a medial collateral ligament injury during the 0-0 draw with Crystal Palace on Sunday.
"He will undergo further examinations in the coming days to establish the extent of his lay-off. We wish Gabriel a speedy recovery."
Gabriel, 20, who left Selhurst Park on crutches, is likely to be sent to the Barcelona clinic of Ramon Cugat, the doctor who treated him when he suffered a fractured metatarsal last season.
Cugat also treated City defender Benjamin Mendy when he suffered an anterior cruciate ligament injury earlier this season.
Kevin De Bruyne also left the pitch on a stretcher and the club hasn't yet given an update on his fitness, but the Belgium international was later seen walking back to the dressing room with two of the club's medical staff.
De Bruyne is likely to miss Tuesday night's game against Watford but sources have told ESPN FC that David Silva and John Stones are both in contention to start the match.
Silva has missed four of City's last five games for personal reasons, while Stones has not featured since Nov. 18 after suffering a hamstring injury
The Watford match will be City's fourth game in just 11 days and Guardiola insists it's not good for players to have such a busy schedule.
"Ask the FA. It's a tradition, so I have to adapt," he told multiple English newspapers. "Last season we played at Anfield, we lost; two days later we played at home against Burnley, we played for 65 minutes with ten men against 11. It is what it is.
"If you tell me that technically, physically, it's good for the players: no, it's a disaster. But the league is the league. They decide at Christmas time to do this, and of course we are going to adapt.
"Sometimes you have three days' recovery, sometimes four, sometimes two, everybody is the same. Last season we had less recovery than this season.
"I can give my opinion but do you believe I am going to get the authorities to change something? No way."