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The Premier League champions lost 1-0 at St Mary's and Alexander-Arnold was at fault for the goal inside two minutes. He failed to cut out James Ward-Prowse's free-kick, which arrived at the feet of ex-Liverpool striker Danny Ings, who lobbed the ball over Alisson.
Alexander-Arnold simply never recovered as he gave the ball away 38 times - a record for this season - before Jurgen Klopp substituted him on 77 minutes.
Trent Alexander-Arnold vs Southampton Highlights ' Generational Talent? Future GOAT RB? :fire::eyes: pic.twitter.com/z2Ys00NeYc- my name is babajimmy (@JMNDComps) January 4, 2021
"I'm not even mad at this because he's been woeful," commented one Liverpool fan on the 'highlights' package, which showed the No 66 being beaten one-on-one by Southampton players and giving the ball away several times.
"My god I didn't realise it was THIS bad," replied another Twitter user.
I remember pool fans saying he is better than Dani Alves https://t.co/i1sMIuQkP1- Rudra Makkar (@MakkarRudra) January 5, 2021
I'm not even mad at this because he's been woeful- ً (@BabyKeith8i) January 4, 2021
Early 2010s Stoke would love this guy ffs- Chiddy :flag_ng: (@chiddyafc) January 5, 2021
My god I didn't realise it was THIS bad- Dillon ॐ (@DilTee_) January 5, 2021
The 22-year-old, widely regarded as one of the world's best right-backs, is clearly struggling for form at the moment. And Klopp didn't hide the fact that Alexander-Arnold was responsible for Southampton's goal, scored after just 111 seconds.
"The goal was already defended but we have a player who is able to hit the ball and he missed it," Klopp told Sky Sports post-match per the Daily Mail. "All these things can happen, we're all human beings and it can happen anywhere at any moment.
"With set pieces we knew that they do it, we have a player that missed the ball. We could have defended it."
Bruno Fernandes (30) has more tackles won in the premier league than Trent Alexander-Arnold (18)- Ronaldo Brown (@ronaldobrown_98) January 4, 2021
Former Reds legend Jamie Carragher expanded on Alexander-Arnold's first-half struggles, saying: "Trent finds himself in a two-v-one even though Liverpool have the numerical advantage. It's something they [Southampton] have worked on. The way [he lost his man] sums up Trent's half.'
Fellow Sky Sports pundit Paul Ince added: "Alexander-Arnold makes a big mistake, he should be doing better."
Trent Alexander-Arnold vs Southampton:- WhoScored.com (@WhoScored) January 4, 2021
:white_check_mark: Error leading to opposition goal
:grimacing: Lost possession 38 times - a record in a PL match this season
:repeat: Subbed off 77th-minute
:thermometer:️ WS Rating 5.30 - lowest in nearly 12 months pic.twitter.com/B5UgQzpHLm
The reigning PFA Young Player of the Year, Alexander-Arnold was one of the stars of Liverpool's title-winning campaign last season, registering 13 league assists.
But this season he's registered just two assists and no goals in 15 league games as he's struggled for form.
Liverpool have also taken just two points from their last three games - and their next league fixture is against title rivals Manchester United a week on Sunday.