John Barnes has cast doubt on whether Manchester United can sustain the form that has taken them to the Premier League summit ahead of Sunday's huge clash with Liverpool.
The Anfield battle between the long-time rivals gives United an opportunity to pull six points clear of Jurgen Klopp's men, which is a scenario few envisaged even a matter of weeks ago.
Nine wins and two draws in 11 games means United have soared from a lowly standing to the top of the table, yet Liverpool legend Barnes believes one setback could trigger a slide.
He compared Ole Gunnar Solskjaer's team to Mikel Arteta's inconsistent Arsenal, and expressed confidence it will be Manchester City and Liverpool who finish the season as the top two in England.
"I think it's too early to say for Manchester United," Barnes told Stats Perform News.
"Because of course, remember when Ole first came after 13 games, they felt that everything was okay. Then they went through a period. And of course, they're very similar now, whereby they're being inconsistent, as much as inconsistency can be being top of the league, with going out of the Champions League.
"But I think that they're showing a resilience that even if they don't play well, they can get results, which is the sign of championship contenders.
"But very much like if you look at Arsenal, and winning one or two games and all of a sudden everything's rosy for Mikel Arteta, lose one or two games and everything will be all doom and gloom.
"I think Manchester United are similar. I still see Man City as our biggest challengers for the title, I still think us and Man City will be first and second.
"Man United are in a good moment now. But will that harmony continue, knowing that one or two bad performances or defeats, well then the old questions about Ole again, the old questions about [Paul] Pogba?
"You need to get rid of all of those of uncertainties if you really want to push for the league, but at the moment, they're doing really well."
Barnes added: "We know what we're going to get from Liverpool. But the intangible is we don't know what we're going to get from Man United."
Sunday's game will be Klopp's 200th Premier League match as Liverpool manager, and Barnes says he has "improved them beyond measure", even if the recent dip in form has cast some doubt over their prospects of back-to-back titles.
Two points from a possible nine since Christmas has been a worrying twist in Liverpool's campaign.
Barnes said: "We're going through a phase now but we'll come out of it. We were top of the league until Man United went above us. So if this is what's being called having a blip, whereby we can go back top of the league if we beat Manchester United, long may this continue.
"I think we've just become so spoiled and expecting Liverpool to be scoring every weekend, beating everybody 5-0, that when they don't, everyone sees that as a bit of an issue."
The goals have dried up in the league since a 7-0 drubbing of Crystal Palace on December 19, with the front three of Sadio Mane, Mohamed Salah and Roberto Firmino looking to sharpen up their form.
"I have no fears that those three players will start to score goals," Barnes said. "And we'll be where we should be in the coming weeks and months."