The loss to the Seagulls on Wednesday night was the first time United had led at home by two goals and lost since they were beaten 3-2 by Tottenham in 1976, half a century ago.
After the Manchester derby defeat, the Red Devils were seeking to bounce back under manager Michael Carrick but instead the side fell once more as the United boss comes under increasing pressure.
In the second half, as United threw away their 2 goal lead, Fernandes and Mainoo were brought on to drive the side to grab a winner, but instead Brighton grabbed one of their own and pushed on to the next round of the cup.
Speaking after the final whistle, Redknapp tore into Fernandes, Mainoo and Joshua Zirkzee who he believes only destroyed the side’s chances of turning their performance around.
“They (Brighton) looked better coached, they had patterns of play even though they made changes today they looked like a thoroughly good side,” Redknapp said.
“United threw on their insurance policy in Bruno (Fernandes) and (Kobbie) Mainoo. Everyone thought they might be able to make an impact and in fact it made them worse.
“Zirkzee came on and couldn’t pass the ball to his own teammate. Incredible for Brighton, they deserved it.”
Pressure is building on Carrick and as United head into their next clash against Fulham on Sunday afternoon, a game where, if 3 points are not secured, Carrick may be let go.