Goals from Chris Porter and Dave Kitson won it for the Blades, who parted company with manager Danny Wilson in midweek.
MacDonald said: "There were a few choice words in that dressing room because, not for the first time recently, we've produced a performance that wasn't up to scratch.
"We're a good team and we know we can do much better than that. Now it's up to us to go out there and show it in our next match.
"We showed flashes of it against a United team which we always knew was going to be very fired up because of what has gone on in the week.
"But we've got to put that together now over the course of 90 minutes rather than in bits."