Manchester City striker Sergio Aguero says "everything went well" after having surgery on his injured knee.
The Argentine had travelled to Barcelona to see a knee specialist amid fears his season might be over.
His manager Pep Guardiola said it is too early to put a timescale on the 32-year-old's return to action.
Asked if Aguero would be fit for the resumption of the Champions League on 7-8 August, City's Spanish boss said: "I don't know right now."
He added: "Once he has finished the surgery we will know the time Sergio will come back to play with us."
Aguero was forced off before half-time in the home side's 5-0 Premier League win over Burnley on Monday, following a challenge by defender Ben Mee.
In his post-match interviews, Guardiola revealed Aguero had a knee problem before the match.
City are due to face Real Madrid in the second leg of their Champions League last-16 tie with a 2-1 lead from the first leg in Spain.
On Wednesday, the player tweeted: "Everything went well and I will soon start with the recovery."
Even if Aguero was only missing for six weeks, he would almost certainly miss City's Champions League last-16 second-leg tie against Real Madrid, on 7 or 8 August.
By then, City will know whether they will play in next season's competition.
City had their appeal against a two-year ban imposed by Uefa heard by the Court of Arbitration for Sport at the beginning of the month.
Guardiola is expecting a verdict in the next two weeks.
"I know the sentence will be in the first 10 days of July. After that we will know exactly what the jury said," he said.
Whether City are successful or not will have a significant impact on their summer transfer business, which is already shrouded in uncertainty given the conversations Guardiola has already held with the club's hierarchy.
"We have discussions since month one, month two, month three," he said.
"We see each other every day and talk about the team but this year is completely different to other years, so nobody knows.
"I don't know what is going to happen for so many situations."
'I can only focus on us'
Meanwhile Chelsea manager Frank Lampard says he will have no thoughts on the Premier League title race when his side entertain City on Thursday evening (20:15 BST).
If Liverpool beat Crystal Palace on Wednesday, City will need to win at Stamford Bridge to keep the title race alive for another week before they play Liverpool on 2 July.
"I am aware of the situation, but I can only focus on us," said Lampard, whose side currently sit fourth in the table, four points behind third-placed Leicester, with a game in hand.
"I have had absolutely no thought of Liverpool's situation, it makes no odds to us at all.
"We respect Liverpool and Manchester City as two fantastic teams, in the last two to three seasons they've been dominant, but we can only look at what this match means for us.
"We have a job to do and our job is to go up against one of the best teams in the world over the last few seasons and try to get a result."