Referee Martin Atkinson awarded a controversial goal to Juan Mata, which gave the Blues a 2-0 lead during the match.
Mata struck the ball towards goal early in the second half, but replays showed that it hit a crowd of players who were clearly in front of the line.
"The second goal was a disaster," said Redknapp. "It was nowhere near a goal. It was a huge mistake.
"[Atkinson has] made a big mistake. I don't see how he can give the goal. It's nowhere near over the line.
"There were bodies on the line and the ball couldn't possibly get over the line. He must have [guessed]. He can't have been sure."
Redknapp also insisted that goal-line technology should be brought into the game as soon as possible.
"Goal-line technology has to come into the game," he added. "You can't keep having situations like that."
Chelsea will now face Liverpool in the final in May.