Gary Neville thinks Manchester United face a fight to qualify for the Europa League this season, let alone the Champions League.
United are winless in their last three Premier League matches following a 2-2 draw with struggling Southampton on Saturday.
They are seventh in the table, level on points with sixth-placed Everton but eight behind Tottenham in fifth.
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After seeing Spurs lose 4-2 at Arsenal on Sunday, former United defender Neville told the Gary Neville podcast he thinks Jose Mourinho's side have significant ground to make up on their rivals.
He said: "If you are Manchester United, where are you at? Watching them on Saturday against Southampton they look a million miles away from that [Arsenal and Tottenham].
"United are in a massive battle to get into the top four, even the Europa League places at the moment."
United have the chance to close the gap on one of their top-four rivals on Wednesday as they face Arsenal at Old Trafford.
But, after the Gunners impressed in their win over Tottenham, Neville said: "Manchester United have to prove, more than they have so far this season, that they can compete with this Arsenal team.
"Old Trafford is always a difficult place to go, so it wouldn't surprise me if Manchester United won on Wednesday night, however they will recognise they have had a warning today from Arsenal.
"No team should now be shocked about how Arsenal are going to play in big matches in the future."
United midfielder Ander Herrera said Wednesday's game is now "like a final".
"I know we are far from the top four but we cannot go crazy now about the table," he said.
"The only thing we can do is give everything on Wednesday against Arsenal and then Saturday against Fulham. We should approach the Arsenal game like a final."
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