Arsenal follow their most important match of the season with what could be its toughest match of the season — a visit to defending Premier League champions Manchester United.

The Gunners improved on their poor start to the domestic campaign to qualify for the Champions League with Wednesday’s 2-1 win at Udinese, but tomorrow’s game at Old Trafford will require yet another improvement in quality.

Criticism of Arsenal and manager Arsene Wenger by fans and commentators started over the offseason with the sale of Cesc Fabregas and failure to strengthen the squad.

It intensified with a start to the season that featured a 0-0 draw with Newcastle, a 2-0 defeat to Liverpool and two red cards, but reaching the lucrative Champions League should have at least bought a little breathing space.

“Some people questioned our mental strength, but if you look at the game that’s what we showed,” Arsenal captain Robin van Persie said. “Now it’s up to us to show that every three or four days. We have to do that over and over again, that’s just football.

“We can’t live off this night for the rest of the month or the rest of the year. Now I’m looking forward to Manchester United.”

But Arsenal have lost seven and drawn one of their last eight visits to United, who are in form and share the early season lead after Monday’s 3-0 dismantling of Tottenham.

Co-leaders Manchester City are at Tottenham in another of tomorrow’s four games, while today’s programme includes Chelsea vs. Norwich and Liverpool vs. Bolton.

Arsenal’s poor start threatened to become truly calamitous when Antonio Di Natale’s first-half header pulled Udinese level on aggregate in the Champions League play-off. But Van Persie and Theo Walcott scored and Wojciech Szczesny saved a penalty to take Arsenal through.

“We have been a little bit under pressure and for us not to play the Champions League and with the players who left, that pressure would’ve increased, understandably,” Wenger said. “Since the beginning of the season, I feel a very strong, determined attitude within the team.”

That attitude will be needed against United. Arsenal have regularly dominated the teams’ meetings in recent seasons before failing to take advantage of possession and opportunities against the older, more experienced United side.

But with Paul Scholes retired and other players injured, United have relied on youth so far this season.

Striker Danny Welbeck, midfielder Tom Cleverley and defender Phil Jones all played against Spurs. The trio has an average age of 20 and could keep their places against the Gunners.

Arsenal will be without Samir Nasri after selling him to City, and the France midfielder could make his debut against Tottenham.

“It was a long negotiation and discussion between the two clubs,” Nasri said. “I was a little bit frustrated. I said to the manager I wanted to leave. I wanted to do the preseason with Manchester so I could know a little bit more about my teammates.

“It was frustrating but it is a new start and I am looking forward to doing it.”

Spurs face losing their opening two matches for the first time since 2008, when Juande Ramos was fired as coach and replaced by current manager Harry Redknapp.

Also tomorrow, West Brom host Stoke and Fulham are at Newcastle.