John Hartson believes that losing at home against Rangers is a warning to Celtic that the path to the title will not be easy, with the Scottish Premiership no walkover for the Bhoys this season.
Celtic slipped to a 2-1 defeat at Celtic Park against Rangers, who scored their first win at the ground since 2010 and moved to within two points of Neil Lennon's men, with a game in hand.
Victory would have seen Celtic go eight points clear and also deflated Rangers, and Hartson insists the defeat serves as a warning to the Bhoys.
The former Celtic striker is now hoping that the Bhoys can use the winter break to recharge their batteries as their recent of winning trophies in recent years is superb.
"Today was a warning that we are not walking this league", Hartson wrote on Twitter.
"Hopefully the break will make us fresher when we start back in 3 weeks.
"We are going for 9in a row and 15 titles out of the last 19!
"Phenomenal… nothing handed out in December!"
Celtic are next in Premiership action on 22nd January when they are due to travel to Kilmarnock, while two further league fixtures are also scheduled before the end of the month, against Ross County and St Johnstone.
