Eden Hazard must prove his fitness by returning to full training to feature in Chelsea's Champions League semi-final first leg at Atletico Madrid on Tuesday, according to assistant coach Steve Holland.
Belgium playmaker Hazard has been ruled out of Saturday's Barclays Premier League clash with Sunderland at Stamford Bridge as he continues to fight calf trouble.
Holland still believes Hazard can force a return for Tuesday night's Madrid trip, but admitted the 23-year-old is running out of time.
"He hasn't trained yet, but that doesn't definitely rule him out of the game on Tuesday," Holland said.
"He's obviously close but it's a day-by-day one at the moment.
"For him to be considered for Tuesday, he obviously needs to train with the group very quickly.
"We'll need to see how he is (on Saturday) having completed what he's done (on Friday), and the process continues.
"Normal procedure is that you wouldn't ask a player to play in a game without some training beforehand, so it's a tight period for him to be made available for that game.
"He's got to train with the group in the next couple of days."
Holland said Hazard's unusual calf problem means Chelsea must not take any risks with his recovery, especially with the trip to Liverpool on Sunday, April 27 that could have a huge bearing on the destiny of the Premier League title.
Asked whether Hazard will travel to Spain with the Chelsea squad, Holland said: "That decision hasn't been made yet.
"He's got a calf injury. The injury is very low on the calf.
"Normally if I said calf to you, you would think that bulge just below your knee. It's lower than that, almost at the back of the Achilles, and it's quite deep.
"So it's an unusual calf injury.
"The consideration would be what's best for the individual, first and foremost. If he's not going to be available for Tuesday, if we can give him a better chance of being involved for Sunday, then that would be the criteria that we would work to."
Hailing Hazard's nomination for the PFA Player of the Year award, Holland said the former Lille midfielder has had a "major impact" on Chelsea's campaign.
"Eden has made outstanding progress this season," Holland said.
"There are only three things a player can do when you get down to the nitty-gritty - make goals, stop goals, score goals.
"There's nothing else - it's one of those three.
"I think this year he's improved at stopping goals and he's improved at making and scoring goals.
"So the three productive elements to his performances, there's been a significant improvement in each area.
"Of course he's only a young man. Our expectations are high. It was his first year in the Premier League last year.
"You would hope there would be an improvement this year, and then an improvement again next year.
"He's had a major impact on our season to date."