The Red Devils threw away three points in dramatic fashion, as Luke Shaw handled the ball in the last minute of the game.
Alexis MacAllister stepped up to smash the penalty past David De Gea in the 99th minute.
“Christian Eriksen was a big miss today. When he doesn’t play, they miss that calmness and quality.
“Fred wasn’t as comfortable on the ball in those tight areas and that’s something they really missed without Eriksen.
“They just weren’t good enough today, Brighton were the better team and fully deserved the three points,” Redknapp said on Sky Sports.
Manchester United are away to West Ham on Sunday, while Brighton host Everton on Monday night.