I THINK every Celtic fan knew deep down that there would come a point in time when Kieran Tierney would move on to another club.

But the prospect of him joining Arsenal, who have had an initial £15 million bid rejected, has taken everyone by surprise a little.

Tierney’s love of the Glasgow club is well known and he hasn’t made any noises about wanting to move.

But I am sure joining a club like Arsenal – who are still a top English team even though they haven’t enjoyed as much success in recent seasons as they have in the past – will be tempting for him.

I think they are using the fact that Kieran plays his football in Scotland just now to nick him at a cheap price. Sorry, but £15 million is nowhere near enough. But they will come back in again.

Celtic are rightly looking for a lot more. I think £25 million or thereabouts is a realistic asking price for him. Full backs don’t come cheap down south.

Aaron Wan-Bissaka is set to move from Crystal Palace to Manchester United for £50 million. So £25 million isn’t an insurmountable fee for a club in England to pay for a player in that position.

Can Tierney resist the lure of playing at the top level in England? I think he will look at how Andy Robertson has progressed in the last few years and decide he has to go.

A couple of years back I would have said that Tierney was slightly the better player of the two Scotland left backs. Now Robertson is ahead of him.

That is down to the level of opposition that he is playing against and the standard of footballer that he is playing with in the Liverpool side as well.

Tierney will be asking himself ‘is it time for me to test myself at that higher level?’ He will benefit financially from a move to the Premier League. But I am sure football will be the major factor if he does go.

That said, I’m sure Kieran would be happy to remain in Glasgow and help Celtic to complete 10-In-A-Row. He might end up doing that. He has got a difficult decision to make.

Sometimes in football a player has to make these choices quicker than they had anticipated and that is where he is just now.

I moved from Celtic to Chelsea when I was 26. Kieran is only 22. But he is level-headed enough to cope with the switch. He has also played in enough games at a high level to do it.

If Celtic were playing in a different league they won’t be in this situation. But the lure of the Premier League in England is so great for professionals that it is hard for them to turn down.

Elsewhere, I was pleased to see Christopher Jullien seal a move to Celtic. He is better known in his native France than he is in Scotland, but the fact that it has cost £7 million to secure his services from Toulouse would suggest he is a top player.

That the Parkhead club are prepared to spend that sort of money – as they did last year with Odsonne Edouard – on the centre half shows great ambition.

It also suggests that it is unlikely that Filip Benkovic, who did so well last season after joining on loan from Leicester City, won’t be coming back.

Kristoffer Ajer and Jozo Simunovic finished last season exceptionally well. But with Dedryck Boyata going and Benkovic returning to his parent club it was necessary to strengthen in that area. They have done that with a bit of quality.