AdrianMrowiec failed with a hopeful penalty appeal as he tried to run between two defenders, but United's sloppy passing was allowing Hearts more possession in dangerous areas.
He thought his team lost their way after that goal and admits he gave central midfielders Ian Black and AdrianMrowiec "a bit of a roasting" at half time for chasing forward.
Moments later McGregor had to react quickly to save a darting header from the edge of the box after AdrianMrowiec had pushed up from defence and, unusually, David Weir had been beaten in the air.