Celtic have been exploring the domestic market as they prepare to add in the January transfer window.
Scottish rivals will be bracing themselves for official approaches – especially those who are currently pushing the Hoops hard in the Premiership title race.
Dundee United are only five points off the pace and took three off Celtic prior to Christmas.
Among those currently shining for the Tannadice outfit are Nadir Ciftci and Stuart Armstrong.
Both are said to be of interest to Celtic but, with United having offloaded Andy Robertson and Ryan Gauld over the summer for close to £6 million, the powers that be at Parkhead have been warned that they will have to dig deep in order to get deals over the line.
United chairman Stephen Thompson said in the Daily Record: “We are not keen on selling anybody and don’t have to do so.
“But we understand there is a price for everybody and if a club makes an appropriate offer then we will have a decision to make.
“The important point is we have set the bar at a certain level in the summer and I don’t expect us to move that bar in a downward direction.
“We lost Andrew Robertson and Ryan Gauld but felt satisfied that we had been properly compensated.
“That would have to be the case with any of our players as we don’t have to sell those who are under contract.”
Ciftci netted 17 times for Dundee United last season, following a switch from NAC Breda, and has added a further eight this term.
Midfielder Armstrong has also been among the goals, with his efforts earning senior international recognition with Scotland.
Both men are only 22 years of age, so Celtic will be looking at them as useful additions for the present and future.