Mackay must decide on whether to give Heidar Helguson a new deal.
The striker, 35, has scored nine goals this season and netted just once since the start of December.
Mackay said: "We'll wait until after the end of the season on that. But everyone is playing for something whether you are in contract or not.
"You want to impress every time you go over the white line. My players know they have to keep playing well to be part of something next season which is very exciting.
"I've had to be ruthless from the first day I've been here in terms of the decisions I've made with staff and players. That didn't change last summer and it won't change this summer.
"As long as I keep my eyes front and centre that what we're doing is for the good of the football club. It's football, business and life. Change is constant and you have to go with it. If you don't you are left behind."