Helguson limped off during the 3-3 draw with Leicester last Saturday with a calf injury and had an MRI scan on Wednesday to assess the extent of the problem.
Mackay said: "We're working on Heidar. He's touch and go and we need to see how he reacts to intense physiotherapy.
"He's had knocks and bruises and injuries throughout his career but he's a Viking, he just gets on with it.
"Over the next couple of days there will be a lot of work done on him before we make a decision on Saturday morning."
Nathan Ellington is on standby to replace Helguson but midfielder Scott Severin could remain sidelined with a groin injury.