Marcos Alonso's absence from the 4-1 drubbing against Watford has been put down to a minor hamstring injury.
The Times newspaper revealed that the Chelsea star missed the trip to Vicarage Road after he suffered the slight strain in training ahead of the defeat to Javi Gracia's Hornets.
Eyebrows had been raised among the Blues faithful as to why manager Antonio Conte had elected to leave Alonso out of his starting lineup, but any relief over his unavailability being anything more sinister hasn't helped to dampen the fury over the defeat in Hertfordshire.
The 27-year-old had been an ever present in Conte's Premier League lineups throughout the season, and the reigning champions certainly missed his talents on Monday as they fell to a shock defeat.
Alonso isn't the first Blues player to miss out due a hamstring issue, with the likes of Willian and Ross Barkley also falling foul to strains.
Conte has come in for increasing criticism for failing to make a good fist of defending Chelsea's crown this term, and the heavy defeat to Watford means that it is now just one win in the past five league outings for the Italian and his charges.
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Chelsea find themselves 19 points behind league leaders Manchester City, and could even struggle to make the Champions League if they are reeled in by Tottenham and Arsenal, who sit one and five points behind them in fifth and sixth place respectively.
Their faltering form has led to rumours that Conte will be sacked by owner Roman Abramovich if results don't immediately improve, and Conte will be desperate to have Alonso back as soon as possible.
The Spain international has weighed in with seven goals and three assists this season, with his end product helping Chelsea to scoop up an extra seven points during the 2017/18 campaign.
Davide Zappacosta was utilised as a makeshift left wing-back against Watford but the right-sided defender was given the run around by Darryl Janmaat and Gerard Deulofeu down Chelsea's left flank for much of the encounter.