Ruben Amorim has called into question the quality of Manchester United's academy graduates, with the Red Devils boss taking aim at three fan-favourites. 

United have a proud tradition of players graduating to the first team from the academy, with the likes of Paul Scholes, Gary Neville and Marcus Rashford rising through the ranks to succeed for the senior side.

However, with fans eager to see youth stars representing the first team, Amorim has insisted he won't be swayed by sentiment and singled out Toby Collyer, 21, Harry Amass, 18, and Chido Obi, 18 - three youngsters who have struggled to impress this season. 

Midfielder Collyer appeared to make great strides last season and was tipped to oust Casemiro in the team, but has only started three times at Championship side West Brom. 

Left back Amass is toiling with Championship strugglers Sheffield Wednesday - where off-field matters have hindered the playing team. 

Meanwhile, talented striker Obi has stagnated while playing for the Under-21s.  

'He (Collyer) played (for the United first team). He's from the academy. He played here,' Amorim said when justifying his decision to keep popular midfielder Kobbie Mainoo on the fringes of his first team plans.

'He went to West Bromwich. He's not playing. He played for Manchester United. So sometimes it's not because he's from the academy or whatever.

'It's the situation, what's happened. I don't know what is going to be the future (for Mainoo). Let's see after this match.

'Amass is now struggling in the Championship. Chido (Obi) is not always a starter in under 21s.

'All these guys played when a lot of people were saying sack the manager!'  

Amorim has included academy products Shea Lacey and Jack Fletcher in his recent squads but the duo have not seen any game time. 

Under the Portuguese's tenure, only three players from the academy have been given first team opportunities: Amass, Obi and Tyler Fredricson - the latter has not featured since he was substituted at half time of United's Carabao Cup defeat by Grimsby.  

Elsewhere, Amorim responded to vociferous criticism from club legends this season.  

Amorim has come under fire from Rio Ferdinand, Paul Scholes and Nicky Butt in recent days over his handling of young midfielder Mainoo, who will again look to leave United on loan when the January transfer window opens.

The United boss insists the 20-year-old has not taken advantage of his opportunities, and issued a sobering assessment of the club's other academy stars such as Toby Collyer, Chido Obi and Harry Amass.

Ferdinand, Scholes and Butt have claimed that Mainoo has to leave United for the good of his career after failing to make a Premier League start this season.

'Sometimes they don't have all the information and they see Manchester United with the standards that they lived here, always winning. So it's hard for them to see their club in this situation,' said Amorim ahead of Monday night's clash with Bournemouth at Old Trafford.

'I take that naturally. It's a fact that me as a manager of Manchester United, I think we are underachieving. We should have more points, especially this season.

'The problem is that me, as a manager, I'm not doing good enough and I can accept that. So that is the only problem. Not winning is the issue.

'If I'm winning I can go to the games on a horse, arrive there, play with just two defenders and everything will be fine.

'At Sporting (Lisbon) there was no issue because we were winning all the time. So I understand and that's okay.'

Napoli were Mainoo's preference when United rejected his request to go out on loan in the summer, and the Italians will be at the front of the queue again next month.

'No, I didn't speak with him, not about that,' added Amorim. 'I had some conversations with him, especially last year, and with other players, but not about that subject.

'The team comes first and I will be really pleased if Kobbie comes to talk to me about that. I just want my players happy, and I understand that every individual has their goals.

'The problem is that we are playing with two midfielders and you guys see Kobbie in a different way than I'm seeing him. Maybe if we play with three midfielders, Kobbie will have more minutes.'