The Bees missed out on automatic promotion in dramatic fashion last weekend when Marcello Trotta's last-minute penalty hit the crossbar and Doncaster broke up the other end to score the winner and secure the title.
Striker Donaldson was involved in the heated discussion with Trotta and captain Kevin O'Connor over who should take the spot-kick, and revealed manager Uwe Rosler has since taken steps to prevent another mix-up.
"I didn't know who the penalty taker was," admitted 22-goal top scorer Donaldson ahead of Saturday's first leg at the County Ground.
"I knew Marcello and Kevin were practising them before so I went up to ask who was taking it. I thought it was Kevin but Marcello said it was him. It was very confusing, I don't think the manager made it clear before the game who was the penalty taker.
"I think the manager has learned from that and he will put a sheet up before games to say who is the penalty taker, so that should end any confusion."
Fulham loanee Trotta was the Brentford fall guy but Donaldson says the players have rallied round the 20-year-old forward.
"All the lads have been brilliant, a few rang him on Saturday night and told him not to worry about it," he added.
"But all the best players miss penalties, we've missed a few this season, and we're not blaming anyone.
"Everyone always says it's better to go up at Wembley and I've never played there so it would be a good thing to look forward to."