MacDonald's side suffered a late setback at The County Ground after Kevin O'Connor thundered in a last-gasp spot-kick to wipe out the Robins' one-goal lead, handed to them by Tottenham loanee Massimo Luogno's fine effort.
The former Aston Villa caretaker coach believes it could come down to which team recovers fastest before the clash at Griffin Park on Monday afternoon, a ground which has been something of a fortress for the Bees this season.
"We think we are evenly matched so it will be about who recovers quickly and gets that little bit of luck," he said.
"Whether that is a decision or a bit of magic to win the game.
"The players have to get physically and mentally ready. We hope they'll all be fit and rested up.
"We are still both in the tie and it comes down to this one off game. It is still very exciting and there is a lot to play there.
"There is no clear favourite, even though they have the best home record in the league. But we feel we can play as well away from home as we do at home. We always think we have a good chance anywhere we play."