Mackay was frustrated in his attempts to draft in reinforcements on transfer deadline day having lost Tommy Smith, Tamas Priskin, Mike Williamson and Jobi McAnuff over the summer.
But the emergency loan window opens on Wednesday with Mackay on the look-out for at least three new faces and he wants one in place by the time the Tykes arrive at Vicarage Road.
"I am actively looking. The recruitment process doesn't stop once the window closes," said the Scot.
The Hornets have been liniked with a loan move for their former striker Heidar Helguson, who is currently with QPR.