Celtic hero Chris Sutton insists "there are still question marks " over Brendan Rodgers' side.
The Hoops currently sit top of the Scottish Premiership but faltered in both the League Cup and Champions League.
And Sutton feels the January window will be vital for the Parkhead side.
"I think there are still question marks. Europe was a massive disappointment, we can't get away from that," he told Sky Sports.
"In the league they've shown inconsistency comparing this team to Ange Postecoglou's team where Celtic were fast and furious.
"This season it's been a struggle at times. Brendan Rodgers has talked about getting a better quality of player in but he also talked at the start of the season about understanding the model.
"So January is really one to watch out for. Are they going to go for more experienced players, who have been there and done it and are going to cost a hell of a lot of money?
The former Premier League winner was also keen to point out the summer recruitment was lacking, particularly in the centre-back position.
"Where do I think they need to strengthen?" he continued. "Left-back. And the centre-half situation is baffling.
"I mean, what on earth has gone on there? Scales has taken his opportunity.
"But Celtic spent £7million-£7.5m on Nawrocki and Lagerbielke and they look surplus to requirements.
"That's the truth of it. I wonder about the striking position and how that will pan out.
"There's a lot of talk about wide players coming in but Celtic have loads of wide players."
