Manchester United manager Jose Mourinho is not the type to pass up an opportunity to get a message across, and in the programme notes ahead of Everton's visit to Old Trafford earlier this month, he had two messages.

One was for Everton manager Ronald Koeman: Mourinho suggested that a team which spends £140 million in the transfer window should "at least" finish in the top four.

The other was for the Manchester United fans.

"I do hope our noisy supporters are back from their holidays for today's match," he wrote. "I want the lads to enjoy a supportive atmosphere at all our home games."

He is not the first United manager to question his team's home support.

"The crowd were dead," complained Sir Alex Ferguson after a 1-0 win over Birmingham at Old Trafford in January 2008. "It was like a funeral out there."

Then there was United legend Roy Keane's "prawn sandwich" jibe in 2000.

Mourinho felt the need to gee up the fans during United's 2-0 win over Leicester in August and said supporters could "do better" after the League Cup semifinal first leg against Hull in January.

He hasn't always been critical.

After watching his team beat Middlesbrough with two late goals last season, he said there were "70,000 on the pitch against 11" in the last 15 minutes.

There have been other highs. During Ferguson's time, Champions League knockout games against Barcelona in 2008 and Real Madrid in 2013 rank among the best atmospheres generated inside an English stadium in the last 10 years.

But it is not big European nights or games against Liverpool and Manchester City that are the problem; rather, the problem is re-creating the noise levels for Everton, Leicester and Stoke.

"Let's hope Old Trafford wants us to win," Mourinho said ahead of the Europa League semifinal second leg against Celta Vigo.

"Because when Old Trafford wants, we win, so that's what we want."

He is not the only one who wants a better atmosphere at Old Trafford.

"A lot of people will see his comments and jump to one conclusion, that the atmosphere is totally flat and nothing is being done about it," says Ian Stirling, vice chair of the independent Manchester United Supporters Trust.

"But there are continual efforts from different groups of supporters."

United fans are trying. The club have explored ideas such as fan parks, but supporters' groups would prefer to grow the atmosphere organically.

A "singing section" -- not a term fans prefer -- housed in J Stand, where the Sir Alex Ferguson Stand meets East Stand, is into its fourth season.

Supporters raised £3,500 for a mural of Bryan Robson in the concourse, and a scheme to offer cut-price food and drink in the buildup to kick-off has been piloted to get supporters into the stadium earlier.

The singing section was first trialled at a Champions League game with Real Sociedad in 2013, when fans were given a letter that read: "You know why you're here, to make a difference, to make Old Trafford better."

The letter continued: "The atmosphere at Old Trafford could have been much better over the last 20 years. We all know the reasons why. Loss of terracing is a major one, the cost, singers being spread out."

It is a good summary of how the atmosphere at Old Trafford has gradually been sterilised.

The Stretford Paddock, the Scoreboard Paddock and the United Road, areas of the ground that bounced and swayed in the 1970s and '80s, disappeared under seats when the Taylor Report was published in the wake of the Hillsborough Disaster in 1989.

A block of executive seats installed in the centre of the Stretford End has not helped, either.

There are other issues.

"I'd say the average age of a season-ticket holder is 50," says Stuart Craig, who sits in the Stretford End.

"It's going to be £1,000 per season if we do reasonably well in the cups. Younger people just can't afford that type of commitment.

"I suspect the people who would add most to the atmosphere are being priced out at the moment."

United's travelling support -- considered among the best in the country -- is, anecdotally at least, a younger group.

"For games against the top six, the atmosphere is generally pretty good," says Paul Dowdall, another who sits in the Stretford End.

"But there's still a disconnect between J Stand and the other main singing section in Stretford tier two, if only because of distance between the two and stadium acoustics."

The acoustics of Old Trafford is a problem, but it is not easily solved.

"We had the trial for the singing section in L Stand [where the away fans sit] and it was fantastic and the acoustics were great," says Stirling.

"If J was actually in L, I think the discussion would be different now. I think there is something with the acoustics.

"J Stand had an auspicious start and it didn't really work. But the progress that they have made has been really good. It's like an oil tanker; it will take years to turn it around completely."

With the growing success of J Stand comes an added complication -- the fans, often tourists looking to tick something off the bucket list, who want to sample the atmosphere without contributing.

It is part of the balancing act United face in trying to keep regular match-goers onside while continuing to market themselves to lucrative parts of the world such as China and the United States.

They also need Keane's prawn sandwich brigade to reach postmatch day revenues of more than £110 million each year.

Most United fans have their own ideas about what would improve the atmosphere, but most agree the introduction of safe standing should be the ultimate goal.

United officials have been in touch with their counterparts at Celtic, who installed 3,000 rail seats last season.

"The club are behind it, and they have said they will give their full support," says Stirling.

"I have no doubt at all that when the time comes, United will be one of the first to introduce it. All these atmosphere issues, it will help with those. I've got no doubt about that.

"The club has had a fractious relationship with supporters groups for the past 12 or 13 years, but in the past three or four years, I think we have started to build up a really good relationship.

"The objections to safe standing, whether it's from safety advisory councils, government politicians or councils, there is all that to overcome. From within the club, we've found them exceptionally supportive."

There is hope that a plan can be put in place as early as next year.

Mourinho is not the first United manager to encourage United fans to raise their game and, with it, the roof. He is unlikely to be the last.

And while some fans at Old Trafford fit Keane's stereotype, others want to make a difference.

Their message to Mourinho is: We are trying.