Brad Arthur conceded the Eels "capitulated" as their blazing start fizzled into a straight-sets finals exit.
Parramatta were seemingly in control with an 18-8 half-time lead over South Sydney, but made a plethora of errors and conceded five tries in 40 minutes to bow out 38-24 at Bankwest Stadium.
It was perhaps a microcosm of a season where the Eels won eight from their opening nine matches and then dipped in form to finish third at the end of 20 rounds.
"The boys are disappointed and so we should be because it's a massive lost opportunity," Arthur said.
"It's very disappointing. We put ourselves in the position, we had 40 minutes to get the job done and we didn't. We capitulated for mine.
"The last couple of weeks, the way we've started our second halves haven't been great. We've got to learn that when we come out of the sheds we need to go back and start again and build that game.
Match Highlights: Eels v Rabbitohs
"If we just kept it simple in the second half, completed our sets ... That's where you build pressure. You just can't keep turning up tackling for long, long periods against really good teams.
"In saying that, I thought our edge defence was pretty good on their shapes. But we just got caught out with some soft tries.
"We're getting some dumb moments wrong."
The "soft" tries Arthur referred to may have been South Sydney's efforts to score from an intercept, a fumble and a mid-field grubber kick by Parramatta that went awry at the death.
While his counterpart Wayne Bennett spoke of how the Rabbitohs, who finished the regular season sixth, timed their surge to the playoffs perfectly, Arthur denied that Parramatta peaked too early.
"We had to start [the season] well. I thought we did start well. Obviously we hit a bit of a flat period but I think the energy's been there over the last month or so," he said.
"We just lost our way a bit in the second half."
Arthur said the shock provisional suspension of veteran centre Michael Jennings on match morning for an alleged doping violation did not distract the team.
Jennings's brother George played on the wing and scored two tries while debutant Haze Dunster was solid on the other flank as the pair deputised for the injured Blake Ferguson and Maika Sivo.
Get Caught Up: Finals Week 2
"[George] did a really good job. He managed to brush aside some personal matters and get on with the job for his teammates," Arthur said.
"I thought he did well. I thought Haze did well. I thought for the positional changes that we made, they did their job.
"I called all the boys, every single player this morning. Their attitude was great and obviously probably there was a bit more of a deeper conversation with George.
"But then when he got here he shrugged it off and was ready to go.
"I spoke to Michael earlier today. It wasn't long but it was just around making sure that he's got the support that he needs around him with his family."