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A smattering of boos rippled around Anfield at the final whistle.

The fortress has fallen. Liverpool's nine-month unbeaten run on home turf lies in tatters.

Rather than atone for their woeful display at Swansea earlier in the week, Jurgen Klopp's side plumbed new depths .

Make no mistake, this was an absolute shambles of a performance.

The FA Cup represented Liverpool's best hope of ending a painful six-year wait for silverware but that opportunity was tossed away in galling fashion.

Relegation-threatened West Brom, who have only won one Premier League game since August, were good value for their place in the last 16.

For the third successive season under Klopp, Liverpool crashed out in the fourth round. But this one cuts deeper than those previous early exits to West Ham and Wolves because of the personnel on duty.

This wasn't a second string line up packed full of kids, this was the strongest side Klopp could have picked but to a man they were hopelessly off the pace. There was a lack of dynamism, composure and even basic organisation.

Defensively, it was a horror show. So much for the £75million signing of Virgil van Dijk sorting out that backline .

The Dutchman, who was the hero in the last round against Everton, endured a night to forget. Joel Matip, alongside him, was no better. Communication appeared to be non-existent.

The Reds got off to a flyer courtesy of Roberto Firmino but the wheels quickly came off as Jay Rodriguez twice took advantage of some glaring defensive deficiencies.

Not even a helping hand from VAR (video assistant referee) technology could save Liverpool. First, Andre Marriner spared Simon Mignolet's blushes by ruling out Craig Dawson's header and then the hosts were given a penalty which Firmino hammered against the woodwork.

There was no reprieve when the hapless Matip bundled the ball into his own net and despite a late revival Mohamed Salah's 25 goal of the campaign proved in vain.

Everyone wants to see the right decisions made but VAR needs tinkering with. It took far too long and killed the flow of the game. Supporters inside Anfield had no idea what was going on.

The sense of confusion within the fanbase will be considerably greater if Klopp doesn't bring in any further reinforcements before Wednesday night's transfer deadline.

His declaration that he already boasts sufficient squad depth sounds increasingly hollow after two such chastening defeats.

The bottom line is that Liverpool have sold their best player this month and have failed to replace him. That doesn't sit right.

Fans want to see ambition and the Reds' season, which now hinges on staying in the top-four and having a prolonged run in the Champions League, could depend on it. Philippe Coutinho was a match-winner and Liverpool simply don't have enough of them currently.

Klopp sprung a surprise with his line up as he made just three changes following the defeat to Swansea. Mignolet, Trent Alexander-Arnold and Alberto Moreno were drafted in but most were given the chance to make amends for events in South Wales.

(Image: Liverpool FC)

Liverpool got off to a dream start as they led inside six minutes.

Chris Brunt's backpass was loose and Jonny Evans hesitated, allowing Salah to steal in.

Ben Foster was out quickly to make the block but Firmino latched on to the rebound and clipped a majestic chip over the stranded keeper.

However, the Reds' delight lasted barely 60 seconds. They were so busy congratulating themselves that they clocked off and allowed the Baggies to instantly restore parity.

Gini Wijnaldum and Alexander-Arnold were beaten far too easily and Rodriguez hammered a sweet strike past Mignolet.

If that was poor, what followed simply beggared belief. Grzegorz Krychowiak skipped away from Emre Can and found Kieran Gibbs on the left. The marking was shoddy and his low cross was coolly converted by Rodriguez.

“How **** must you be, we're winning away,” crowed the away end.

Liverpool were all over the shop defensively and bewilderment in the stands swiftly turned to anger.

Moreno was poor on his comeback from injury, while on the other flank Alexander-Arnold endured an equally tough night.

Van Dijk turned his back and needlessly conceded a corner as the ball bobbled behind off him. Brunt swung it in and Dawson got above Firmino to nod home with Mignolet appealing that he had been blocked off.

There was no flag from the assistant but Pawson went to VAR and Liverpool got lucky as the footage showed that Gareth Barry was marginally offside.

A wake-up call? It should have been but alarmingly Liverpool's lethargy continued.

When Salah went down in the box after being tugged back by Jake Livermore, Pawson wasn't interested initially but once again the technology came to Liverpool's aid.

(Image: Alex Livesey/Getty Images)

The game was held up for far too long with Pawson waiting for advice through his ear piece before going over to the touchline to have a look for himself on the monitor. Eventually, he pointed to the spot.

Rather than accept the gift, it was spurned as Firmino's penalty hit the underside of the bar and stayed out.

Liverpool had their moments with Salah thwarted by Evans' fine block before the winger fired into the side netting.

However, there was no control as they repeatedly conceded possession cheaply and made a succession of glaring errors.

They were so weak in the engine room with Can, Wijnaldum and Alex Oxlade-Chamberlain contributing little. Behind them it was even worse and in first-half stoppage time Liverpool leaked a third goal in embarrassing fashion.

Nobody bothered to track the run of Dawson who latched on to Krychowiak's lofted pass. The defender's cross-shot was turned into his own net by the hapless Matip.

Whatever was said at the break had zero impact. Crosses failed to beat the first man and moves continued to break down as passes went astray.

The Baggies, who were fortunate that Ahmed Hegazi got away with a dangerous lunge on Firmino, could have finished the tie off.

Twice, Matt Phillips raced away on the counter but he failed to beat Mignolet and then he lashed wide.

Can's miserable night was summed up by the sight of him picking up a senseless yellow card for bringing down Gareth Barry.

The anxiety levels in the stands continued to grow with Mignolet getting it in the ear after taking too long over a goal kick.

Finally, midway through the second half the changes arrived as Can, Oxlade-Chamberlain and Sadio Mane got the hook.

Captain Jordan Henderson, Danny Ings and James Milner were sent on together as Klopp played his hand.

Belatedly, Liverpool started to exert some real pressure and the Kop finally had some hope to cling to. Milner's strike was clawed away by Foster, who then pushed away Ings' effort.

With 12 minutes to go the Reds halved the deficit. Alexander-Arnold's cross bounced off Firmino and into the path of Salah. The Egyptian clinically dispatched it past Foster.

In doing so he became the quickest Liverpool player to reach 25 goals in more than a century.

Van Dijk could have salvaged a replay deep into stoppage time but nodded too close to Foster.

Anfield left Klopp in no doubt about their verdict on a desperate night.

So Swansea wasn't a one off. A season which promised so much is stuttering badly and the manager has plenty of food for thought ahead of deadline day.

MATCH FACTS

Liverpool: Mignolet, Alexander-Arnold, Matip, Van Dijk, Milner, Can (Milner 65), Wijnaldum, Oxlade-Chamberlain (Ings 65), Mane (Henderson 65), Salah, Firmino.

Not used: Ward, Lovren, Gomez, Solanke.

West Brom: Foster, Nyom, Evans, Dawson, Gibbs (Hegazi 37), Barry (Yacob 71), Krychowiak, Livermore, Brunt, Rodriguez, Robson-Kanu (Phillips 39).

Not used: Myhill, Rondon, McClean, Burke.

Referee: Craig Pawson

Attendance: 53,342

Goals: Firmino 6, Rodriguez 7, Rodriguez 11, Matip OG 45, Salah 78.

Bookings: Barry, Can, Dawson, Hegazi.

Man of the match: Mohamed Salah. Was surrounded by mediocrity.