Scunthorpe boss Brian Laws and his players blamed a momentary lapse in concentration for their 1-0 defeat at Brentford.
Bees subsitute Tom Adeyemi broke the deadlock in Saturday's League One clash at Griffin Park with his first touch, coolly side-footing home from 10 yards with 20 minutes left after good work by Marcello Trotta.
Laws said: "I'm very disappointed to come away with nothing because I thought for long spells we really deserved something from the game.
"We know Brentford like to play, but we frustrated them and were always in the game.
"There wasn't a lot between us and Brentford which shows that as a group we are going in the right direction."
Iron defender David Mirfin said: "We were surprised when Donaldson was not in the squad, but it didn't change our game-plan.
"We defended well and it was probably a second's lapse that let them in for the winner."
But Laws, whose side are near the top of the League One form table, added: "We showed we can compete with one of the best in the division when we work as hard as we did.
"There will be a lot more 'cup finals' to come for us this season."