The Pompey captain was stretchered off the field and taken straight from Fratton Park to hospital.
However, scans have revealed that the former Blackburn Rovers midfielder has not suffered a broken leg.
Pompey boss Steve Cotterill told the club's official website: "It's a big relief that Aaron's okay. He had a scan and has taken a bad whack on it but there's no break.
"It's been hard enough to get the sort of squad we have together without losing anyone to serious injury.
"It looks doubtful that Aaron will be available for Tuesday night's game at Crystal Palace, but you never know."
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