The five-time European Cup winners collide with their fierce rivals in the final of the International Champions Cup in the United States following the two sides' victories in their respective groups.
The North-West rivals square off in Miami's Sun Life Stadium, and ahead of a season that includes a Champions League campaign, the England international believes it will be a good test for his side.
Although United won't be involved in Europe's premier competition this coming season, Johnson feels that a game against Louis van Gaal's men will be a decent yardstick to judge his team's capabilities heading into the new term.
He told the club's official website: "When you're at a top club, you need to be playing the best teams because they're the ones you're going to be fighting with throughout the season. It has been a fantastic pre-season.
"It will be great for the fans and we're certainly looking forward to it. They're a massive club as well so I'm sure the fans will turn up and make it a good night.
"Although it's pre-season and the main thing is to get fit, we've started the way we finished last season. That has pretty much been the philosophy for the whole time.
"We're playing Champions League sides in every game, so it's brilliant for keeping your level where it needs to be and keeping fitness levels high. It's looking good."
Brendan Rodgers' men confirmed their participation in Monday's final with a 2-0 win over Serie A side AC Milan on Saturday, as goals from Joe Allen and Suso saw off the threat of a limited Rossoneri.
The win was the Reds' third of the tour after a penalties triumph over Manchester City came hot on the heels of a 1-0 victory over Greek champions Olympiakos - although they slipped to a last-minute defeat to Roma on July 25.
Rodgers' charges will meet Bundesliga giants Borussia Dortmund on Sunday in their final pre-season warm-up before they begin their campaign with a visit from Southampton on August 17.