The City manager watched his side go down 3-2 to a late Brentford winner in their latest pre-season game at Griffin Park.
Mackay has so far brought in striker Andreas Cornelius and goalkeeper Simon Moore, and afterwards he told Sky Sports News the search for new recruits is ongoing.
He said: "The group we have are at the fitness level we expect them to be at, so that's important. but at the same time I still need to add two or three players.
"We've got various plates spinning. We're working hard on it but they've got to come in and add to this group.
"We're looking at good players to come in and we're trying to improve in all areas."
Mackay stonewalled when asked whether his list of targets includes Celtic's Joe Ledley, Sunderland midfielder Jack Colback or Tottenham's England defender Steven Caulker.
But he did say Joey Barton's presence at Griffin Park was no more than a coincidence.
"I think he lives round here," Mackay said.