As if Celtic fans don’t have enough reasons to dislike Atletico Madrid already.

Boy, that escalated quickly. From a few whispers here and there about reported interest in Matt O’Riley from the likes of Internazionale and Girona, there has now come a bolt from the boys in blue shorts.

Atleti have reportedly lodged a bid for Celtic’s midfield diamond, who they have polished up in fine style since picking him up from MK Dons for a relative pittance a couple of years ago, looking to take O’Riley to the Spanish capital on a loan basis now with an obligation to buy him in the summer.

Celtic fans may initially scoff at the notion that their club would allow arguably their star man – and certainly one who could be pivotal to their title defence - to leave on loan at this juncture of the season.

But the crucial details here lie in what that promised fee will be at the end of the campaign, and what the effect that news of interest in him from a club of such stature does to O’Riley.

READ MORE: Paulo Bernardo on his Celtic future and goalscoring streak

I wrote in this column just a couple of weeks back that keeping the 23-year-old would be Celtic’s most important piece of business of the transfer window, and that it should take something approaching or perhaps beyond their club record sale to even consider allowing his departure in January.

I stand by that, and in fact, the delay in paying that fee should push the asking price even higher. If they can get towards the region of £30m, even if it isn’t coming until a little bit down the line, that may well prove too good for Celtic to turn down.

If they do allow O’Riley to leave, what they must do though is invest a substantial amount of their bulging bank balance into the team, and bring in the quality that manager Brendan Rodgers has been crying out for in attack and at left back, as well as bringing in a replacement for O’Riley.

Despite widespread feeling to the contrary, the Celtic board are savvy enough to know that the backlash from fans - and most likely their manager - would be brutal if they do not then deliver on those fronts, and they can do so safe in the knowledge that the club’s bank balance would be topped up again down the line.

Reinvesting sufficiently in the team would give them the best chance of earning that all-important Champions League cash as well on top of the O’Riley fee. On the flip side, should they miss out on the former having banked the latter, there will be hell to pay.

Sure, you can argue that taking the guaranteed promise of say, £30m, isn’t worth it if you are likely to command somewhere in the region of £25m in a summer sale anyway. What is £5m if there is a greater chance of winning the league title and securing group stage Champions League football?

But if O’Riley does depart Celtic in this window, accusations towards the board of penny-pinching ahead of on-pitch success won’t paint the full picture. Celtic manager Rodgers himself has often said that he won’t look to keep a player at the club if they are not happy to be there.

There is nothing to suggest that O’Riley is not happy to be at Celtic, for the record. And until this news broke, he seemed hugely content with his station at this stage of his development.

Offers from one of the top clubs in Europe though don’t come along all that often, and as sad as it is to say, Atletico operate in a different stratosphere to Celtic these days. With the greatest of respect, this isn’t a Wolves or a West Ham. It is hard to see how a player’s head wouldn’t be turned by such interest, and why they wouldn’t want to pursue the opportunity.

And if he isn’t already, this move would also make O’Riley the perfect poster boy for Celtic’s oft-derided recruitment model. They could rightly say that when it comes off, it comes off handsomely.

If it does come to pass though, what Celtic must not do is look at the recent win over Rangers, the emergence of Paulo Bernardo as a midfield force, factor in the return of Reo Hatate in a couple of weeks’ time and decide that all is rosy in the garden.

Atletico’s interest is a game-changer in many ways, and if they do stump up the asking price for O’Riley, one of them has to be the recalibration of the targets that Rodgers can aim for.

Nobody is expecting Celtic to reinvest the entire fee or pay anything like such a figure on a single player, but they do have to make some kind of statement, particularly with Rangers showing a willingness to spend in pursuit of Celtic during this window. Another raft of projects at £2m each ain’t going to cut the mustard.

Trouble is, they don’t have much time to do business, so Celtic fans had better hope the contingency plan assistant manager John Kennedy insisted was in place a couple of weeks ago should O’Riley depart is already swinging into action. And that it doesn’t consist of applying a hairdryer to a deep-frozen James McCarthy.

READ MORE: Celtic the perfect path as Caoimhin Kelleher stands at Liverpool crossroads

There is a week of the window left, and sans O’Riley, Celtic would undoubtedly be in a weaker position than they were coming into it, having only added Nick Kuhn to their ranks so far.

Whether the loss of O’Riley would sting as much as 1974 and all that, or the 6-0 drubbing Celtic took in Madrid earlier this season, time will tell. But it seems unlikely the interest from Los Colchoneros in O'Riley will soon be put to bed.

Celtic must be prepared to react accordingly.