Returning manager Martin O'Neill says Celtic will go "all out" to sign players in January. O'Neill has been hired at Celtic Park for the second time this season and is embarking on his third spell at the club overall.

His appointment until the end of the season was confirmed shortly after Wilfried Nancy's sacking on Monday, the Frenchman dismissed following six defeats in eight games in charge. "I think that we would have to supplement the squad in some capacity," O'Neill told TalkSport. "We'll have to do that if it's at all possible. January transfer windows are difficult, difficult to get the players that you probably want at the football club but we do need to supplement it and we're going all out to try and do that."

O'Neill, 73, says the demand to win the Scottish Premiership has "not been something that has been put to me" by the club. He won seven out of eight matches in charge after replacing Brendan Rodgers, before Nancy's December arrival. Celtic and Rangers trail league leaders Hearts by six points, with O'Neill's side hosting Dundee United on Saturday.

"It'll be very, very difficult. We're behind, Rangers have caught up now as well," O'Neill said. "It is a challenge for us. In this day and age, it's instant results."

The Northern Irishman, who first managed Celtic from 2000-05, explained his latest return to the club came in "very similar circumstances again". "Obviously, the results just recently for the football club haven't been great but even though it was still a bit of a shock," he said.

"Very, very difficult to say no to the man who put me in charge a way back 25 years ago [major shareholder Dermot Desmond]." O'Neill, who will again be assisted by Shaun Maloney and Stephen McManus, insists he "genuinely" does not know what went wrong under Nancy and also played down the prospect of an extended spell as Celtic manager.

"I can't see that for a start," he said. "I genuinely can't. I don't know whether I would have the energy and all that type of stuff. I'll just put everything into it now for these next couple of months and the same applies to me - if I don't win football matches, I'll be under the same pressure and that starts here at the weekend."