The Republic of Ireland international is set for a spell on the sidelines after sustaining the injury in his side's defeat to Manchester City at Villa Park on Sunday
The center back had to be replaced in stoppage time after falling awkwardly following an aerial challenge inside the City area with goalkeeper Joe Hart.
Villa boss Alex McLeish confirmed the extent of the injury after the match, telling reporters: “He’s not going to be back for the next game, that’s for sure. I don’t know at this moment exactly what’s happened but he’s definitely damaged his shoulder.
“I do believe he is going to be out for at least a month.”
McLeish went on to defend his tactics against Roberto Mancini’s side, which moved back to the top of the table thanks to a second-half goal from Joleon Lescott.
The hosts’ best chances at Villa Park came in the closing stages in a match which City dominated, but the Scot believes his team was unlucky not to take anything from the game.
He added: “We had a game plan and I thought the players did really well in executing that. If you open up early and you leave yourself wide open against City they will destroy you.
“You saw Everton’s tactics a couple of weeks ago, very similar to what we deployed tonight. And you’ve got to keep City at nil to have a chance to win the game.
“I think we were the unluckiest team in the league this weekend.”
McLeish also confirmed that defender Ciaran Clark will be out for “a few weeks” with a knee ligament injury.