Chelsea returned to winning ways in their first appearance after the World Cup break, but delight at their 2-0 home victory over Bournemouth on Tuesday was tamped down by a new injury concern for Reece James.
The England right-back lasted only 53 minutes on his return from the knee injury that ruled him out of the World Cup, leaving Chelsea fearing the worst as they await a more detailed examination and prognosis.
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"He's not good, but we'll have to wait and see. It's too early for me to speculate, but whenever he comes off like that, it is not positive," manager Graham Potter said.
Eight minutes into the second half, James signalled to the bench that he needed to come off before going down in some discomfort. He looked distraught as he laid on the pitch at Stamford Bridge receiving treatment, but did walk off on his own.
The 23-year-old had surgery after suffering a knee injury against AC Milan in the Champions League on Oct. 11 and it was the knee that looked to be the cause of his anguish again.
"We'll have to keep our fingers crossed and find out in the next day or two," Potter said.
Chelsea, who had not won in their last five league matches and started Tuesday's game ninth in the standings, were rejuvenated with James at right-back as they scored twice in the opening 23 minutes to secure the three points.
James took no time at all to remind of his ability, especially as Chelsea swept forward with quick pace in a convincing performance.
On the eve of the match, Potter had said James was in a "good place" after the "challenging period" of missing out on the World Cup.
"It was a World Cup and they come around every four years. So when you're young, the perspective is not so easy to get," Potter said. "I have to say he is in a good mood, he is in a good place.
"He has trained with us, is training well and is a good teammate. He has dealt with it well."