Manchester United's results are not good enough after the investment made in the team, but top director David Gill called Sunday for fans to stop jeering under pressure coach Louis van Gaal.

Facing a new onslaught after a 1-0 home defeat by Southampton, Gill, a former chief executive who remains on the United board, said: "Clearly we all hoped the results would have been better and everyone is disappointed with that.

"Undoubtedly it has been a season of underachievement given the investment that was made in the summer."

Gill told the BBC that the boos aimed at Van Gaal and his team as they left the field on Saturday were the worst he has seen.

"No one likes to see that. We're fans as well as directors and we don't want booing, we want cheering. But I think we've got to stay calm."

Manchester United have spent £250 million ($355 million) on players since Van Gaal arrived in 2014 but after the latest defeat they are fifth and five points adrift of a top four Champions League place.

Van Gaal has been particularly criticised for United's style of play. Saturday was the seventh time they have failed to score at Old Trafford this year.

Van Gaal accepted the supporters criticism after what he called a "poor match."

"I know that everyone, from the owners and Louis Van Gaal and his team, is working extremely hard to turn that around. It's not easy but we have to stick together," Gill said.

The Old Trafford torment has fuelled reports of meetings with Pep Guardiola and Jose Mourinho writing a letter to United leaders.

Both have been strongly denied however.

Mourinho's agent rubbished an Independent on Sunday report that the Portuguese coach wrote a six page letter to Manchester United setting out why he should be their next coach.

"It is absolutely ridiculous and totally absurd," Jorge Mendes said in a statement on his website.

Mourinho, free since being sacked by Chelsea in December, wrote a detailed analysis of how United could be overhauled and how he would adapt his style of management to suit the Old Trafford style, according to the Independent on Sunday.

The 52-year-old Mourinho has been linked with the job before, but United chose David Moyes to succeed Alex Ferguson in 2013 and Van Gaal to come in a year later.

Gill declined to comment on the Mourinho report but did say he does not believe Van Gaal is finding the United job more difficult than he imagined.

Van Gaal is having to come to terms with a more competitive Premier League, said Gill.

"He's managed at the top level, at the top clubs in Germany, Holland and Spain, I don't think he's found it that difficult but the sheer competitiveness of English football is there for everyone to see."