United's 2-1 victory over Chelsea on Sunday took them to within one point of a record 19th top-flight title but that has not stopped some pundits questioning their quality in relation to previous title-winning teams.

With Barcelona widely considered to be among the best club sides of all time, the Wembley clash on May 28 will give the Red Devils a perfect chance to state their case.

Former United midfielder Macari told Press Association Sport: "I don't think people should be judging the team before the season's over.

"If they go to Wembley and play Barcelona off the park, which I hope they do, and win the game comfortably, then you can take stock.

"Week in, week out trying to compare them to teams of the past, I don't think it's right. It's almost impossible to do."

Macari, who is promoting E.ON's Energy Fit Fans experience ahead of the FA Cup final between Stoke and Manchester City this weekend, is also reserving judgment on Barcelona, although he expects United to be up against it.

He said: "I think they can beat them but I do believe they'll have to play the best they've played all season because they're playing against the best team they've played against all season.

"Barcelona look good and we see them from a distance, (Lionel) Messi without a doubt is brilliant, but let's see how they do at Wembley before we make any real judgments about them."

United need only a point from their final two games against Blackburn and Blackpool to wrap up their 12th Premier League crown since Ferguson took over the hotseat in 1986.

That would take them above bitter rivals Liverpool in the all-time list with the prospect of a third European title under Ferguson still to come.

Macari added: "It's a great achievement. I thought last Sunday at Old Trafford they did it in style, not only winning the game but with probably one of the best performances at Old Trafford this season.

"Against Arsenal seven days earlier they were awful but that's typical of Manchester United and typical of the manager that, when the biggest games come around and they need to win, he gets them ready to do that and they normally perform.

"No one goes into management expecting the success he's had. It's an unbelievable achievement to do what he's done with different players every three or four years, and he's done it consistently.

"It's the hallmark of one of the best managers, if not the best manager, in the world."

:: The Energy Fit Fans truck and kinetic dance floor mat will be at Olympic Way, next to Wembley stadium, from 3-7pm on Friday and before the final on Saturday, giving Stoke and Manchester City supporters the chance to win £10,000 for their club's Community Trust Programme. Fans can also join in and win final tickets at www.eonenergyfit.com/facup.