Idrissa Gana Gueye could miss crunch Everton matches against Manchester City and Liverpool because of a muscle tear.
The midfielder had linked up with the Senegalese national team during the March international break but, according to his Senegal boss Aliou Cisse who was quoted in the Liverpool Echo, was sent back to Merseyside with the issue.
That leave Gueye as a major doubt for the home game against the Premier League leaders and the Merseyside derby against the high-flying Reds seven days later - a factor that will have Sam Allardyce tearing his hair out in L4.
Cisse told the press: "My squad is almost complete, but Gana had a muscle tear when he came over. I had wanted him to be in my team, but I think he will be out for three weeks.
"I wanted him to come over with the rest of his national teammates, so we could assess his injury. He was able to do so."
Gueye had only returned to first-team action for the Toffees in last weekend's 2-1 away triumph over Stoke City after he had sat out the 2-0 home victory over Brighton a week earlier.
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That absence against the Gulls came about due to a slight bout of illness that the 28-year-old picked up on the eve of the contest, but this latest setback could have even bigger repercussions on the Blues' end to the season.
Gueye has been instrumental in the heart of Everton's midfield since his £7m switch from Aston Villa in the summer of 2017, but has had his struggles this term as the Merseysiders failed to make a fist of gatecrashing the top four with their £200m outlay almost 12 months ago.
The tireless midfield star will now hope to fit enough to feature in Everton's final five fixtures of the current campaign if he is not fit to take on Pep Guardiola's men or Jurgen Klopp's charges.