Everton manager Roberto Martinez has admitted to being left "frustrated" by Alex Oxlade-Chamberlain's challenge on Phil Jagielka.
The Arsenal midfielder caught his England teammate with a late tackle despite the whistle having already been blown.
Jagielka left the Emirates Stadium on crutches following his side's 2-1 defeat, with Martinez unhappy at the way Oxlade-Chamberlain needlessly lunged in.
"It'll take probably 48 hours to find out about Phil, but it's not good, clearly," he is quoted as saying by the Liverpool Echo. "Jagielka never leaves the playing field. It's a problem in his right knee. We don't know at this point what extent the injury is.
"Some of the structure is going to be damaged, but we don't know what it is yet. To lose Jagielka in such a stupid challenge is disappointing, because the referee has already awarded the free kick. It was an unnecessary challenge. Needless. Oxlade-Chamberlain puts his foot up when the referee has already given the free-kick.
"It's an unfortunate action. It's not malicious or with any intent. If you have a player who lunges his body to try and stop a ball going over the goal-line and that ends up with an injury, then you take it. But this is frustrating."
Jagielka has played 14 games for club and country so far this season.
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