It is part of life for Arsenal supporters to be in an almost permanent state of confusion.

A stunning victory, such as the 5-2 victory at previously unbeaten Leicester is followed by an abject display when losing 3-2 at home to Olympiacos, which had tended to travel as well as Nigerian wine in the Champions League.

Nobody knows which Arsenal will turn up at Emirates Stadium for Sunday’s match against Manchester United.

One of the few predictable factors about Arsenal is its unpredictability. A side that has Premier League consistency has the wrong type of consistency in Europe — in three of its last four home games it has conceded three goals to Olympiacos, Monaco and Anderlecht, who are hardly Champions League heavyweights.

It would be dangerous to bet against Arsenal winning the Premier League, yet defeats by Olympiacos and Dinamo Zagreb with two encounters against Bayern Munich up next leaves the Gunners’ hopes of reaching the knockout stages hanging by a slender thread. They are staring down the barrel of the Europa League.

Arsene Wenger, unusually, lost his cool when questioned about playing David Ospina, whose error allowed the Greeks to take the lead, instead of Petr Cech. “I know things you don’t,” he said without offering any explanation. “I don’t have to take a poll about who I should play.”

Wenger had said the game was a “must win” so to not play your strongest side is baffling. It is one thing to give the back-up goalkeeper some playing time in domestic cups, but not in the Champions League.

On television, Roy Keane said: “If you were playing, you would fancy yourself most weeks against this group. I enjoy watching them, but they are mentally weak, soft with no leaders.”

Keane’s former club United is top of the Premier League and got its Champions League progress back on track with a 2-1 win over Wolfsburg with Juan Mata outstanding. Their results have been more impressive than their performances, though, and had the Germans drawn at Old Trafford it would not have been a huge injustice.

The game at Emirates Stadium will provide another chance for Anthony Martial to show his potential. In a month the French teenager has become United’s biggest goal-threat, his pace, strength, composure and technique giving a sense of anticipation whenever he is in possession. He is a star in the making.

Louis van Gaal’s biggest problem is how to compensate for the loss of left-back Luke Shaw. He moved Matteo Darmian over from the right with Antonio Valencia replacing the Italian, but the Ecuador international was at fault for Wolfsburg’s opening goal.

Wenger will probably opt for Theo Walcott’s pace rather than Olivier Giroud’s strength at center-forward, but with Arsenal expect the unexpected.

Strange choice: Jack Grealish has made himself available to play for England rather than the Republic of Ireland. The Aston Villa midfielder was born in England, but qualifies for Ireland — he has represented it on 20 occasions from Under-17 to Under-21 level — through two Irish grandparents.

For reasons known only to himself, it has taken Grealish, 20, a year to make up his mind. A clue as to how he reached his decision could be the priorities of his agent, Jonathan Barnett, who also represents Gareth Bale, the Wales international who qualified for England through a grandmother.

Barnett recently told the Soccerex convention in Manchester that he tried — unsuccessfully — to persuade Bale to pledge his allegiance to England. The agent said: “I tell you that it has cost him millions and millions of pounds. You can imagine what it would have been like if he were playing for England next summer in the Euros, but he does love playing for Wales.”

Grealish has yet to establish himself in the Villa team and I can think of at least 13 good reasons why he might regret turning down Ireland: Jordan Henderson, Jack Wilshere, James Milner, Fabian Delph, Ross Barkley, Jonjo Shelvey, Ryan Mason, Alex Oxlade-Chamberlain, Raheem Sterling, Dele Alli, Andros Townsend, Adam Lallana and Michael Carrick.

Good luck, Jack, with what Barnett hopes will be a lucrative international career with England. You’ll need it.

Illogical move: A penny for Mark Warburton’s thoughts on the Rangers manager’s previous club, Brentford, would be money well spent.

Warburton performed something of a football miracle guiding the side from League One to fifth place in the Championship in a year, the Bees losing out on promotion to the Premier League in the playoffs.

Midseason, with Brentford defying logic by maintaining its unlikely push for promotion, owner Matthew Benham decided Warburton was the wrong manager for the club. It was a decision that nobody but Benham agreed with or understood.

Benham’s love for Brentford cannot be doubted and he has pumped £90 million into the club over the past three years. A city investor and professional gambler, Benham believes that stats are the key indicators to how good a player is rather than results or his manager’s view.

Warburton, who conducted himself with professional politeness while effectively serving his notice, did not agree with Benham’s algorithmic philosophy, so a parting of the ways at the end of last season was agreed. Yes, even if Warburton had led Brentford to the most incredible of promotions to the Premier League he would have left.

Rangers are top of the Scottish Championship with eight wins from eight games. Brentford are 19th in the English Championship and Warburton’s successor, Marinus Dijkhuizen and his assistant Roy Hendriksen, have been fired after nine matches. They arrived as strangers and left as strangers.

Benham has put academy director Lee Carsley, who has no managerial experience, in charge of the team. The owner has made no public statement on the latest managerial change. Why he pressed the self-destruct in the way he did remains mystifying and while Brentford fans are grateful for Benham’s financial support, which includes a new stadium, there remains a collective shaking of heads about the owner’s treatment of Warburton.

Christopher Davies was a longtime Premier League correspondent for the London Daily Telegraph.

Correction, Oct. 4, 2015:

The venue for Sunday’s match between Arsenal and Manchester United was incorrectly stated on second reference as Old Trafford. The match was to take place at Emirates Stadium.

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