"I'll certainly be trying [to make a signing], it's not easy for various reasons," Mackay told BBC Three Counties Radio.
"Clubs are cautious letting players out for 28 days. If there was a 24-hour recall it wouldn't be a problem."
Midfielder Drinkwater and striker Weimann only picked up knocks and should be available for the upcoming visit of Millwall.
But midfielder McGinn is likely to join defender Lloyd Doyley on the injury list, while captain John Eustace will serve a one-match ban after receiving two yellow cards at Doncaster.
"Stephen's in a leg brace at the moment and it's something over the next few days that will be assessed," Mackay added.
"There will be a point where our physio department will know more.
"Three of them went in together, he, Adam Thompson and one of their own players, and Stephen's come out of it quite badly.
"It's not something we can afford at the moment.
"It's been a real grind for the group, it's getting smaller by the day at the minute.
"We finished up with with seven academy products on the pitch and quite a lot of teenagers."
Mackay added that youngster Adam Thompson was feeling the effects of sickness and diarrhoea after the draw, which leaves the Hornets eight points adrift of the Championship play-offs.
But Watford are close to having wingers Michael Bryan and Will Buckley back to full fitness.
Source: BBC Sport
Source: BBC Sport