Hendrick was substituted just over an hour into the Clarets' 1-0 victory at St Mary's and could be a doubt for the trip to Copenhagen.

"We have one or two who played over the weekend carrying bumps and bruises, and we'll see how they are over the next few days," said O'Neill at a press conference.

"Jeff came off after about 60-odd minutes, he was having a bit of a problem and he's still a little bit sore at the moment.

"He's still hopeful and I think it's a case of the other players being nursed through the next couple of days as well.

"He's said he's improving so that's good news. If the game was tomorrow morning he might struggle but he's pretty hopeful and the medical team are hopeful."

Injuries have already ruled out defender Richard Keogh, midfielder James McCarthy and forward Sean Maguire while David Meyler is suspended for the first leg.

O'Neill and Ireland have recent play-off experience under their belts, having qualified for Euro 2016 over two legs against Bosnia-Herzegovina.

Denmark missed out in the same play-offs but O'Neill says they have improved since then, with Tottenham's Christian Eriksen an obvious dangerman.

"If you look at the opposition Denmark were in play-offs against Sweden while we were playing Bosnia, and they were disappointed, maybe Zlatan Ibrahimovic was the difference between the two teams," added O'Neill.

"But they have improved a great deal since then and they have some very talented players. Eriksen has been playing brilliantly.

"But our approach will be similar to the game in Bosnia. To make sure we keep the tie alive and make sure it's all to play for in Dublin."