The 25-year-old defender was acquired by the Cottagers over the summer as they replaced his former club Swansea in the Premier League.
It took £20 million ($26m) to secure his services, with the talented centre-half having proved his worth in the English top-flight and made the senior Three Lions squad ahead of the 2018 World Cup.
He has, however, been restricted to just 14 appearances this season.
Mawson has been sidelined since December 29, with a mystery knee complaint keeping him out of action.
Ranieri has now lifted the lid on why he is unable to call upon a key man in a relegation dogfight, with the Italian admitting that the cause of an untimely knock is “unbelievable”.
“He needs more time. How long? I don't know,” Ranieri said when asked for an update on Mawson’s fitness ahead of a London derby date with Tottenham on Sunday.
“It was a very strange injury. He just changed his boots. It was unbelievable.”
While Mawson is unavailable due to injury, Aboubakar Kamara has been shunted down the Fulham pecking order.
Ranieri has taken that decision in the wake of reports suggesting that he was involved in a bust-up with Aleksandar Mitrovic during a yoga session just a couple of weeks after the pair argued over the taking of a penalty at Huddersfield – which Kamara subsequently missed.
“It wasn't a battle, it was a chat and something that happens in every team, nothing more,” said the Cottagers boss.
“I don't think it was about the penalty.
“Kamara is not in the squad. I prefer to think about it and then I will do the best for the team.”
Fulham will welcome Spurs to Craven Cottage sat inside the Premier League drop zone and with just one win collected from their last 10 games in all competitions.