Celtic are preparing for the worst as they await the results of a scan that Colby Donovan went through after he was referred for tests.
The right-back is one of the two left standing amid injury troubles, with Alistair Johnston and Julian Araujo remaining on the sidelines. With Donovan joining them, that leaves Anthony Ralston as the only option.
Johnston was close to his return but suffered a setback during his time with the Canadian national team. O'Neill said last week: "Alistair has had a wee bit of a setback. He went away with the Canadian national side, was doing some running and had a bit of a setback on his final day of training, which was really disconcerting for us."
Donovan could now be ruled out for a couple of weeks, and with no news on when Araujo or Johnston will be back, the reality of Ralston having to step up is seeming more and more likely by the day.
Give Ralston the chance to impress
Celtic supporters should hold out hope for Ralston. Although he may not be the best option at the club, he is one that we are going to need to look to. There is no choice, so the least we can do is try to be hopeful about it.
He should hopefully see us through the final pre-split game and post-split fixtures. However, it seems that we could have both Araujo and Donovan back for the all-important matches against Rangers and Hearts at Paradise, which is a massive boost.
Araujo has been incredible since he joined up with Martin O'Neill's side, but picked up an injury after Celtic's penalty win against the Gers at Ibrox. He returned to his parent club, Bournemouth, for treatment, but will hopefully be on his way back up the road shortly.
Meanwhile, Ralston has made 29 appearances this season. He last featured against Dundee, when he was brought on in the 67th minute to replace Donovan.
He made four defensive contributions but didn't win most of his ground duels. He committed two fouls, but also made mostly accurate passes.
That is something to work with for Celtic, and Ralston will be hopeful that the manager has faith in him. A run of games in the squad will be amazing for the player, as he will undoubtedly be keen to play some part in Scotland's World Cup run in the summer.