Martin O’Neill is hoping his Sunderland side can intensify Arsenal’s pain in the FA Cup – but has warned unhappy Gunners fans that under-pressure Arsene Wenger is the greatest manager the club has ever had.

The Sunderland boss, who has crossed swords with Wenger on the touchline in the past, insists his opposite number should not be condemned for the midweek 4-0 pasting by AC Milan in the first leg of their Champions League last-16 tie.

The FA Cup now looks Arsenal’s only chance of ending their seven-year trophy drought this season, but O’Neill believes his players can reverse the result of last weekend, when the two sides met at the Stadium of Light and the visitors won 2-1.

O’Neill said: “The Milan defeat has been heavy for them and I think they would admit that and that it was something they didn’t expect.

“AC Milan played very, very well indeed and Arsenal on the night didn’t do so.

“Regardless of that I think their concentration will be on the FA Cup tie as it’s a competition they have won reasonably recently and believe they can win.

“AC Milan are a top-quality team and, let’s face it, we are not as good as AC Milan. They are playing in the last 16 of the Champions League and that’s where every single football club worth its salt is aspiring to get to and we are a long way off that.

“That doesn’t mean we are not capable of winning the match, and that we don’t have one or two players who can definitely cause them problems.

“I am just hoping that the advantages we might take into the game, such as having had an extra rest this week and playing at home, will help us.”

O’Neill said Arsenal have found it tough to fill the gap left by Cesc Fabregas and Samir Nasri after their summer departures, and that the loss of faith in Wenger by some Gunners’ fans is unwarranted.

He added: “I think that is par for the course these days. His influence on English football has been immense and if he left tomorrow morning he would be considered Arsenal’s greatest-ever manager – and there have been some excellent ones.

“Much has been made of the fact they haven’t won a trophy for however many years it is but during that time they have qualified for the Champions League each year.

“They have lost some influential players this season that would be hard to replace immediately. But that’s not to say that they don’t have quality players or are not capable of winning this cup.”

Arsenal won 2-1 last week thanks to a last-minute volley by Thierry Henry. but Sunderland will not have to worry about the French striker making a similar impact from the bench tomorrow as he has now returned to the New York Red Bulls.

O’Neill, however, does not believe that Henry’s absence will have a major change on the outcome of the match – and pointed out that he came on to replace Alex Oxlade-Chamberlain – one of the success stories of Arsenal’s season – last weekend.

“Henry has been a fantastic footballer, one of the best players to have ever played in the Premier League, and when he came on maybe I should have thought it would be written in the stars for him to score, but I didn’t think anything more of it,” O’Neill said.

“But he was replacing a very, very good player who I think will be a top-quality player in years to come. So if one good player exits and another excellent player enters I still think you will have problems.”

O’Neill admitted the state of the pitch at the Stadium of Light was “disappointingly poor” and needed major improvement for next season.

Asked if that would be a disadvantage for Arsenal, who struggled on a terrible surface in the San Siro against Milan, O’Neill replied: “You are assuming that they are a better passing team than us so I will let you get on and assume that.”

Meanwhile, Arsenal defender Per Mertesacker has undergone surgery on his ankle injury and will be out “long term”, according to Wenger.

Mertesacker suffered ligament damage during last Saturday’s 2-1 victory at Sunderland and could be sidelined for the rest of the season.

“Unfortunately he has had surgery and we have lost him for a while,” Wenger told the club’s official website, www.arsenal.com.

“How long I don’t know. He had reconstruction of his ligaments, so it will be long term.”