N'Golo Kante was the first name on the team sheet for Chelsea's most recent title winners but now the Blues have a new undroppable player: Mateo Kovacic.
In recent weeks, the Croatian has grown into the player that many expected him to become from the moment Inter signed the midfielder from Dinamo Zagreb in 2013.
After a tough first season on loan from Real Madrid, Kovacic has become Frank Lampard's only untouchable at Chelsea, which only makes the Achilles injury sustained by the 25-year-old in Tuesday's FA Cup win over Liverpool all the more concerning.
Kovacic was forced off just three minutes before the break after a collision with Sadio Mane and although the Blues battled their way to a deserved 2-0 victory, losing the midfielder for an extended period of time would be a calamity for Lampard.
The timing certainly couldn't be worse, for both the player and his club.
Kovacic struggled under Maurizio Sarri last term and Chelsea's decision to make his loan deal permanent arguably had more to do with the club's transfer ban than his performances on the field.
The arrival of Lampard, then, was key to Kovacic's development.
"I have good communication with him," the Croatia international said last week. "I am working on my bad sides of my game.
"I am talking to him a lot and I am analysing my game. I think I am becoming a better player but it is normal because I am getting more mature. I am a good age so I feel good."
Lampard has certainly been impressed by Kovacic's improvement. Indeed, it was telling that he was the only player singled out for praise by his manager after the chastening 3-0 Champions League loss to Bayern Munich.
"Kovacic played with a personality and a quality that makes you go, 'Okay, he can play in pretty much any team with that level'," the Blues boss enthused.
Hardly surprising, then, that Kovacic is firmly in pole position to win Chelsea's Player of the Season awards.
Lampard will, therefore, be mightily concerned by the potential loss of one of his most reliable performers, not least because Chelsea are already short on options in midfield.
Kante will remain sidelined by injury for a further fortnight, while Jorginho is suspended for the next three matches.
The fit-again Ruben Loftus-Cheek racked up 90 minutes for Chelsea's Under-23s in Monday's 1-0 win over Everton, whom the Blues face next in the league before games against Aston Villa and Bayern Munich.
Chelsea also have injury issues out wide, with Willian having also limped off against Liverpool. It's not yet known when Christian Pulisic will recover from the muscle problem that has kept him out since New Year's Day and while Callum Hudson-Odoi could return this weekend, the England international hasn't trained for a month.
Elsewhere, first-choice striker Tammy Abraham has already missed a month of football with an ankle issue, while a minor knock kept centre-half Andreas Christensen out of the Liverpool game.
Chelsea's injury crisis has arrived at a most inopportune moment, with Lampard's side already struggling to hold on to fourth place in the Premier League.
Manchester United and Wolves are now only three points further back, while a win for Carlo Ancelotti's Everton on Sunday would put them in contention for a Champions League place despite currently sitting in 11th in the standings.
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The Blues, of course, only have themselves to blame in this regard. After a bright start to the season, they have failed to rack up back-to-back wins since November and have been particularly poor at home.
Still, Lampard has been understandably buoyed by the impressive cup win over runaway league leaders Liverpool.
Ross Barkley put in his best performance of the season so far, while 18-year-old Billy Gilmour was the star of the show, walking away with the man-of-the-match award thanks to a midfield masterclass that drew praise from one of the Scot's childhood idols, Cesc Fabregas.
"I have been learning off [Kovacic and Jorginho] in training and watching them in games," Gilmour said after the match. "To play alongside both of them was brilliant.
"Hopefully, Kova is back and fit for the weekend."
That now seems unlikely, with Lampard subsequently admitting that Kovacic's Achilles problem looks "worse" than Willian's.
He will be praying that's not the case, though. Of all the players currently struggling with injuries at Chelsea, Kovacic is the one that Lampard needs back in his starting line-up as soon as possible.