Ralf Rangnick is a "complete imposter" who looks like a school teacher and is "embarrassing" to watch manage Manchester United, according to Jamie O'Hara.
United endured a poor week in the Premier League, drawing 1-1 with Burnley and Southampton.
Rangnick's side took the lead in the first half of both matches before conceding an equaliser straight after the break to drop four points.
They are fifth in the Premier League, a point adrift of West Ham, and are in a five-way race for the top four, with Arsenal, Tottenham and Wolves all challenging for the position.
Rangnick took over from Ole Gunnar Solskjaer in November on an interim basis and has so far struggled to implement his tactics, having switched to a 4-3-3 after his customary 4-2-2-2 formation didn't work.
The German manager was plucked from a job as head of sports and development at Lokomotiv Moscow after spending the majority of the past decade as a sporting director and former Tottenham midfielder O'Hara thinks he's out of his depth.
"For me Ralf is a complete imposter at the football club," he said on talkSPORT.
"I don't know what this guy is doing there. He looks like a school teacher on the sideline.
"I remember having someone like him manage me at Dartford district, you know it was that bad.
"I mean the players have seen right through this guy, it's embarrassing to even watch him on the sideline.
"I've never seen anything like it at Manchester United. They're completely flat, they're completely lost and the players have got no confidence in the manager, so it's showing on the pitch.
"You've got no confidence in them as a team. It's a bunch of individuals. Get a proper manager in there, a top manager in there, I guarantee you you're going to see better performances and better results."
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Rangnick is in charge until the end of the season when he will move into a two-year consultancy role with United.
The search is already under way for a long-time successor, with Paris Saint-Germain boss Mauricio Pochettino understood to be leading the race, ahead of Ajax's Erik ten Hag, Sevilla's Julien Lopetegui and Spain manager Luis Enrique.
O'Hara believes that, while some of United's players are not performing consistently, the responsibility ultimately rests with Rangnick.
"You look at that squad of players. Some of them are not having good seasons and some of them are still weak points, but there's weak points in every team," he added.
"Sir Alex Ferguson had weak players in his side. He won the league pretty much every season and you look back at some of the players that played and you'd go 'pff'.
"That 2008 side, Wes Brown played right-back and they talk about that being the greatest Man Utd team or the greatest season they've had. Don't get me wrong he [Brown] was a top player, but he was a centre-half.
"So Manchester United there's always going to be a couple of players that aren't quite there in terms of what you expect Man Utd to be but you get the best out of them, that's what your job is as a manager. But this guy can't get a tune out of any of them, he's a shambles!"
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