The Munster skipper last played in Europe in January after suffering a groin injury in Ireland's RBS 6Nations defeat at the hands of Scotland in March.
He made his comeback for Young Munster two weeks ago, before returning to Magners League action as a substitute in Munster's victory over Cardiff Blues.
He was again selected among the replacements for the red army's 22-16 Heineken Cup triumph over the Ospreys. But O'Connell elbowed Jonathan Thomas and saw red just 11 minutes into his cameo appearance.
It was the first red card shown to a Munsterman in Europe, in 112 games and O'Connell could be facing a suspension. Under IRB regulations, rule 10.4 (a), striking another player with the elbow carries a minimum two-week suspension and a potential maximum of 52 weeks.
"When I saw the card I was hoping I was colour blind but unfortunately I'm young enough that my eyesight is till good," said Munster head coach Tony McGahan after the game.
"Right now I'm keeping my distance from Paul and giving him some time because he's pretty disappointed."