It is alleged that the player was the victim of verbal abuse by three Rangers supporters while he waited in a lounge at Glasgow Airport.
This came in the aftermath of Celtic’s 3-2 victory at Ibrox earlier in the day, as the Ladbrokes Premiership leaders extended their lead at the top of the Ladbrokes Premiership table to nine points.
For Hartson, it’s bad enough to shout and swear at an opposing player inside a football ground, but there’s no excuse when the hatred spills out into everyday life.
He told the Evening Times: “I have to say that I never experienced too much negativity when I was playing for Celtic. There was the odd shout in a pub or a restaurant or one or two disagreements but that was the extent of it.
“But there seemed to be a fair amount of things thrown onto the pitch towards players on Sunday and you need to have a bit of common sense.
“The vast majority of Rangers fans will be appalled to hear that their fans were behaving like that towards Scott Sinclair because it is embarrassing. It is a game of football, that’s it.
“I always hear the argument that punters pay their money and are entitled to shout and swear and say whatever they like but the bottom line is that there is still a way you need to behave. And if you think you can get away with it in a mob mentality inside a stadium then fair enough but when you are living your life and with your family then it is a pretty poor indictment of some people.”