Pompey are left to pick up the pieces of a shattering delayed reaction to manager Harry Redknapp's departure after irresistible Watford sent them the same way as Saints - just when the Carling Cup was promising a route into Europe.
Just 21 days after Southampton were battered 5-2 in the fourth round at Vicarage Road, Pompey took a similar pasting, 3-0, in the quarter-finals of the Football League's flagship knockout competition.
New executive director Velimir Zajec refused to discuss why his new team flopped so badly - he also declined to talk about Saturday's shock win at Bolton.
Even Watford boss Ray Lewington found it difficult to explain why his hard-up Coca-Cola Championship team had again become the scourge of the elite after Icelandic striker Heidar Helguson, pictured, had scored twice and then helped set up a third goal for big Bruce Dyer.
"We took our chances well, worked hard and, when we needed it, saw our goalkeeper Richard Lee make some great saves," he said.
Lewington added: "Portsmouth have had an unsettled time with Harry and Jim Smith leaving and although they had a great win on Saturday we hoped that might work in our favour. Perhaps that is what happened, but we seem to produce our best for these cup contests."
Watford will now meet either Arsenal or Manchester United or ex-midfielder Lewington's former club Chelsea when the draw is made tonight.
Pompey coach Kevin Bond admitted: "It was a magnificent opportunity lost for us. We gave away a poor first goal and the two in quick succession after half-time really killed us.
"We had plenty of possession but goals change matches. We have to admit, though, that we never got in behind Watford and we could not stop them doing what they wanted to do. They did it well.
"The only thing to do now is try to regain a bit of pride at Fratton Park against West Brom on Saturday. I am sure this is a huge blow to our supporters and we need to make up for it. Everybody in the dressing room knows that."
Bond, Zajec and Joe Jordan tried three at the back and six in midfield last night - then switched to 4-4-2 after the break. Neither looked like paying off once keeper Jamie Ashdown's rush of blood had given away the first goal to Helguson in 22 minutes.
The Icelander added another, touching in a Jermaine Darlington cross 12 minutes into the second period and then played the ball which gave James Chambers another shooting chance three minutes later. Ashdown pushed it away but only as far as Dyer, who slotted comfortably home.
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