Theo Walcott struck a hat-trick and Danny Welbeck marked his first start since May 2016 with two goals and an outstanding individual performance as Arsenal thrashed Southampton to reach the FA Cup fifth round. In a tie played out between two much-changed line-ups, Arsenal ruthlessly outclassed Southampton as Claude Puel’s side came crashing down to earth after reaching the EFL Cup final with victory at Liverpool on Wednesday. Welback, back in the starting line-up after that lengthy absence following knee surgery, put Arsenal in control with two cool finishes inside the first 22 minutes then set up Arsenal’s third for Walcott before half-time. The Gunners, with manager Arsene Wenger watching from the stands as he starts a four-match touchline ban, heaped further misery on Southampton after the break, sending on Alexis Sanchez as substitute to create two comfortable finishes for Saints product Walcott.

This was Welbeck’s first start since May 2016 but there was not the slightest sign of rustiness as he settled to his task as if he had never been away. Welbeck’s end product has occasionally drawn criticism but it was as smooth as silk here, taking Lucas Perez’s pass to loft a clever finish over Southampton’s stand-in keeper Harry Lewis for his first, drawing Wenger out of his seat in the stand in delight. He was at it again seven minutes later, bringing Alex Oxlade-Chamberlain’s floated pass under his spell before showing composure once more to beat Lewis. Welbeck then showed his awareness and value as a team player to cross for Walcott to react before any Southampton defender for a simple close-range strike for Arsenal’s third. Welbeck is due a run of good fortune with injuries and he demonstrated here that he brings tireless effort and work-rate to Arsenal’s team as well as a goal threat. If he can finally put a run of games together after his long absence, he will be a key asset for Wenger. Wenger wisely withdrew him after 64 minutes with the game safe, to a fully deserved standing ovation from the travelling fans. There was a mood of celebration around St Mary’s as supporters arrived for this FA Cup fourth-round tie – and rightly so after the outstanding victory over two legs against Liverpool sent them into the EFL Cup final against Manchester United at Wembley on 26 February.

Saints’ boss Puel pointed clearly to his priorities with 10 changes from the team that won at Anfield, only defender Jack Stephens keeping his place in the starting line-up. And how it showed as Southampton played like strangers to be ripped apart by Arsenal. The party mood was deflated but Saints fans were in forgiving mood, with even the smattering of boos at half-time and at the end sounding very half-hearted, as if this team selection and Anfield hangover was to be expected. Wenger watched this emphatic Arsenal display from the comfort of a seat in the stand at St Mary’s. He was serving the first game in a four-match touchline suspension after accepting a Football Association charge of misconduct for verbally abusing and pushing fourth official Anthony Taylor in the win against Burnley at Emirates Stadium.

Wenger, who was also fined £25,000, barely had a moment of discomfort as his right-hand man Steve Bould took charge on the touchline. There is no doubt the Gunners’ boss would have preferred to be in his usual position on the touchline – but he was able to sit back and relax as Arsenal cruised into the FA Cup Fifth round. Theo Walcott scored his fifth Arsenal hat-trick and was the first Arsenal player since Nicklas Bendtner against Leyton Orient in March 2011 to score an FA Cup hat-trick. Welbeck scored and assisted in the same game for the first time since September 2014 against Aston Villa. Arsenal won an away game by five clear goals for the first time since August 2009 against Everton (6-1). The Saints recorded their heaviest home FA Cup defeat since February 1910, when they lost 5-0 to Manchester City. This was Southampton’s joint-heaviest defeat at the St Mary’s Stadium, equalling their 6-1 loss to Liverpool in the League Cup last season. Lucas Perez has been involved in eight goals in eight starts for Arsenal in all competitions (five goals, three assists) and assisted Welbeck’s first goal in this game. Southampton have conceded twice or more in eight of their past nine FA Cup matches against top-flight opponents, doing so in each of their past five. Walcott has scored seven goals in his past four away FA Cup matches. Alexis Sanchez has had a hand in 11 goals in 10 FA Cup games for Arsenal (five goals, six assists). Walcott was the first player to score an FA Cup hat-trick against the Saints since Paul Rideout did so for Tranmere in February 2001, in their famous 4-3 victory.