Mauricio Pochettino has admitted Tottenham are "starting to be concerned" about Erik Lamela's mystery hip injury.
The Argentina playmaker will undergo a scan on Friday on the injury that has kept him sidelined since October - and that is yet to be diagnosed.
Lamela has been working with a physiotherapist at former club Roma in a bid to ease the problem, but Spurs boss Pochettino admitted it is now becoming a real worry.
"Tomorrow he will have a scan," said Pochettino.
"It's still difficult to give a time for him to come back. We need to wait for what happens tomorrow. There are still some problems, and we're not sure of the diagnosis.
"So we need to wait for tomorrow. Now we are starting to be concerned about him, because it's nearly three months and we're still in a situation with no movement."
Asked if Lamela will need an operation to resolve the problem, Pochettino continued: "Today nobody advises that. It's better to wait until after tomorrow."
Tottenham will take on Manchester City at the Etihad Stadium on Saturday, bidding to set a club Premier League record of seven consecutive victories.
Pochettino has challenged Spurs to prove to themselves that they can be genuine title contenders, while also revealing Jan Vertonghen's ankle injury is not as serious as first feared.
Spurs worried defender Vertonghen would miss three months, but now Pochettino hopes the Belgian centre-back will only be missing for six weeks.
"Jan is okay, he's very positive, we are pushing him a lot," said Pochettino.
"We expect him to be out for six weeks, less than we first thought. I don't want to set a limit or the time, but I think that's very positive.
"When your mind is good, positive, you can even recover early."
Second-placed Tottenham could open up a six-point gap on Pep Guardiola's Manchester City this weekend - but Pochettino has demanded his men focus more on their own concerns than their rivals.
"We have to try to show that we can be real contenders for the Premier League, that's the real challenge," said Pochettino.
"It's more important that we can show ourselves, rather than show Manchester City.
"We need to show ourselves we are capable of dealing with that pressure, and to try to win games to achieve big things.
"I think it's not important today the gap that we can take (this weekend).
"The most important thing will be our performance - to show that we have improved a lot and learned from last season.
"It will be a very tough game, I think when you play against a team like Manchester City you must always give your best.
"To try to get the three points, that's our challenge.
"But it's too early to talk about decisive games, or decisive points. There are a lot of big things ahead."