The Foxes are sending chief scout Alex Garden to Ibrox today to watch how the midfielder fares against Rangers. The 29-year-old rejected a new contract offer from Aberdeen in January, although has not ruled out the possibility of extending his contract at Pittodrie.
"It would be a massive blow if he left. Barry was back to his best on Wednesday.
"He is such a good player and he has so much quality when you are playing well. If you give him the space he can get into double figures every season. He is a good finisher and good in the air for his size.
"We will start looking at things when we have time after the Celtic game," said Calderwood.
Nicholson is not the only Aberdeen player whose future is up in the air with Lee Miller, Steve Lovell, Stuart Duff, Richie Byrne and Derek Soutar all coming to the end of their contracts. In addition, loan stars Josh Walker, Sone Aluko, Dave Bus and Alan Maybury will all return to their clubs in the summer.