Calderwood said: "Players have to look after themselves and decide what's best for them.
"At Aberdeen you have a realistic chance of getting into Europe every year - Kevin (McNaughton) hasn't at Cardiff. That's a choice you have to make.
"No disrespect to Kevin, I would have loved to keep him but financially it was impossible and Cardiff is a big club.
"We also tried to get Hammell from Motherwell when he was on the verge of the Scotland squad but he went south for much more money.
"I'm convinced if he had come to Aberdeen and Kevin had stayed here they would be a lot closer to the national team now.
"I've spoken to the guys out of contract in the summer and they know where we are coming from.
"We need an answer definitely before Christmas and hopefully in the next two or three weeks.
"You can only try to sell your idea to them but I don't get the feeling any of them want to leave.
"The handful like Nicholson, Severin, Diamond, Hart and Jamie Smith have the quality to go higher but if everything is right for them and they can see where the club is going I'd be surprised if they leave.
"Obviously finances come into it and if clubs with more clout come in it could be tempting. But it's a hard one to leave Aberdeen because for most of them it's downhill.