Harper spent 20 years at Newcastle as a player, only leaving in the close-season to join Premier League newcomers Hull City.
Few understand Newcastle United as well as him, and Harper – who has had first-hand experience of Ashley's reign – has urged him to reward the club's long-suffering supporters.
Newcastle last won a major trophy in 1969 and struggled last term, failing to build on their fifth-place finish in the 2011-12 season.
Ahead of his testimonial at St James' Park, Harper told The Telegraph: "The fans make the club tick, but they feel detached from it.
"Whenever the club has done well over, the fans have been on board. They have been fully behind their team, their club.
"I think their loyalty has been tested far too often in recent years. It's one of the great clubs, but it should be competing amongst the very best in Europe.
"It's getting harder and harder to break on to the top table and opportunities have been missed. We finished fifth a couple of years ago, but to sustain that you need investment in the team.
"If I had a message to Mike Ashley it would be to engage with the fans, to repair that relationship. It's not totally broken, but it doesn't just need a plaster on it, it needs to be fixed."
Harper is fully aware of the expectation placed on the Newcastle players by an expectant home crowd, and feels things could be even tougher this season.
"If things go well, it (playing for Newcastle) is the one of most amazing experiences you can have as a footballer playing in England," he said.
"When there is a good feeling, when the momentum builds, you swim with the tide and you feel unstoppable. But when things turn sour, when the club is divided, you're in a rip tide and it's incredibly hard.
"The fans are demanding, even under (Kevin) Keegan and Sir Bobby Robson, they'd get restless if you weren't winning at half-time, but all they really demand is you put a shift in.
"They are working class people who want to see players put in the same shift they do in the week to put money in their pockets."
