Jim Ratcliffe, the co-owner of faltering English Premier League club Manchester United, has criticised some of the club's players as "not good enough" adding "some probably are overpaid".

The British billionaire, who took charge of footballing operations at United in February last year, referenced Rasmus Hojlund, Andre Onana and Casemiro, plus Jadon Sancho and Antony, who are both on loan, as he spoke about "players we are buying this summer, that we didn't buy" in an interview with the BBC.

Ratcliffe said: "These are all things from the past, whether we like it or not, we've inherited those things and have to sort that out.

"For Sancho, who now plays for Chelsea and we pay half his wages, we're paying Stg 17million ($A35m) to buy him in the summer."

Asked if he was suggesting those players were not good enough for United, Ratcliffe said: "Some are not good enough and some probably are overpaid, but for us to mould the squad that we are fully responsible for, and accountable for, will take time."

After United - currently 14th in the Premier League - finished eighth last season and won the FA Cup, they opted to stick with boss Erik ten Hag.

The Dutchman was subsequently sacked in October, being replaced by Ruben Amorim. Ratcliffe, chairman of petrochemicals giant Ineos, accepted the decision to keep Ten Hag on was a mistake - and that the same applied to hiring Dan Ashworth as sporting director, a role he left in December.

"I agree the Erik ten Tag and Dan Ashworth decisions were errors," Ratcliffe said.

"I think there were some mitigating circumstances, but ultimately they were errors. I accept that and I apologise for that."

Meanwhile, Ratcliffe announced the club planned to build a new 100,000-seat stadium next to the existing Old Trafford.

"Today marks the start of an incredibly exciting journey to the delivery of what will be the world's greatest football stadium, at the centre of a regenerated Old Trafford," Ratcliffe said.

"Our current stadium has served us brilliantly for the past 115 years, but it has fallen behind the best arenas in world sport.

"By building next to the existing site, we will be able to preserve the essence of Old Trafford, while creating a truly state-of-the-art stadium that transforms the fan experience only footsteps from our historic home."

The club unveiled architect designs for the new stadium at an event on Tuesday morning in London.