Donaldson echoed the sentiments of Bees supporters who taunted the Blues with chants of, "Champions of Europe, you're having a laugh", after going within seven minutes of an astonishing giant-killing act at Griffin Park on Sunday.
And the forward had no doubt his side could complete the job on February 17 in front of Chelsea supporters who have shown no mercy to Benitez since his controversial appointment as interim manager.
"Obviously, the fans are unsettled with the manager," he said.
"They don't really like the manager so anything little thing that goes wrong, they are always on their backs.
"I think that has played a part in their recent results so hopefully we can put that into our advantage and take the positives and unsettle their fans again at their place."
Chelsea lifted the Champions League just eight months ago and there are two divisions and 44 league places between them and npower League One Brentford.
But having watched them throw away an unprecedented five pieces of silverware already this season, Donaldson and his team-mates were anything but overawed.
"We watched all the games leading up to this game," he said. "I felt, if we got our game plan right, we would be able to contain them."
He added: "You couldn't really see that they were European champions.
"We set out to unsettle them. I think we did that and it showed.
"They didn't really create that many chances, which I am very surprised about to be honest, especially with the quality of players they have got.
"We have done our homework on them and we stopped them creating through the middle as well, which is their strong point.
"So I think we have to take some credit out of that."
Donaldson also took confidence about how easily he managed to rattle Ashley Cole, who he was tasked with containing on Sunday.
"He was always mumbling something, always mumbling stuff quietly but I could hear what he was saying," he revealed.
"He was just getting frustrated. He was saying about the forward players not showing and, 'he needs to be there'.
"You could just see straight away that he was getting frustrated - that was a good thing."
The same could be said for all of Chelsea's players.
"You could see how they started the game," Donaldson said. "They didn't really start as fast, they didn't really break us down to be honest.
"Obviously, the pitch wasn't the best so that didn't really help as well. That was for both of us.
"Coming here is a difficult place. We don't really lose many games here. We made it difficult, we are hard to beat.
"For their changing rooms and stuff, it's not the biggest, it's not Stamford Bridge."
And Donaldson could not wait to run out at the home of the faltering European champions, despite ruing a missed opportunity to beat them on Sunday.
He said: "Definitely disappointed to come in with a draw but we have got a great tie away at Stamford Bridge, which we all the boys are looking forward to."