Despite admitting his relief that Bouma - who has had his ankle set in plaster - has not fractured his leg, O'Neill recognises the serious extent of the injury and confesses the left-back will be missed.
"The doctor is back from the hospital and says it's a dislocation," O'Neill told Villa's official website.
"Thankfully the skin isn't broken, which is really good news. They've re-set it and he's now in plaster.
"It's obviously a serious injury to a player who's been playing exceptionally well - he had a great, great season last year and he will be missed.
"The players are going to visit him tonight and we wish him all the very best and the injury is a big blow to us."