Things went from bad to worse for Everton, who fielded new signing Theo Walcott from Arsenal, when 27-year-old McCarthy sustained his injury after the interval as he tried to prevent West Brom's Salomon Rondon from shooting.
The news will resonate strongly with Ireland and Everton fans, following Seamus Coleman's leg break against Wales past year. I think it very much depends on the specialist or surgeon that deals with it.
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"James has fractured his tibia and fibula", he told reporters.
As Rondon followed through, it snapped McCarthy's leg. "It's a big blow for him and us", Allardyce said, adding that the midfielder would be operated upon as soon as possible.
"Now this is one serious injury which we know will keep him out until next season at some stage, that's a big blow".
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Update 5.05pm: James McCarthy suffers apparent broken leg.
In the 58th minute, McCarthy's attempts to block a shot resulted in West Brom striker Rondon kicking his opponent's calf at full strength instead of the ball. The injury left the Everton central midfield requiring oxygen on the field.
The Toffees' official account tweeted shortly after accident: "Heart-breaking moment for James McCarthy as he is stretchered off with what looks like a broken leg after a last-ditch challenge".
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TV directors opted not to show replays of the injury.
The match played on and finished in a 1-all draw.
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