The Manchester United squad leaves for Australasia on July 8 with their first game against Perth Glory on July 13.
It’s reported that United had planned to sign Longstaff in June.
However that was not possible due to negotiations with Crystal Palace over the Wan-Bissaka deal dragging on.
The Longstaff deal could still drag on with United also interested in Leicester defender Harry Maguire.
The Red Devils are targeting a centre-half after a poor season last campaign.
Man Utd chief Ed Woodward given next transfer deadline by Ole Gunnar Solskjaer (Image: GETTY)
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Man Utd chief Ed Woodward given next transfer deadline by Ole Gunnar Solskjaer (Image: GETTY)
United finished sixth in the Premier League and were disappointing for large chunks of the season.
And Solskjaer recently expressed his desire to focus on youth when it comes to new signings.
He said: “We've tried to educate our players and our kids to be proper people. In my view, the best players have always been the best people.
"You have to have that value system and a certain kind of attitude about you to make a Man United player.
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Manchester United struggled last season and finished the Premier League season in sixth (Image: GETTY)
“You have got to have the qualities and our fans want to see exciting players.
"Of course we want to get up from our seats. We want to see defenders who defend like they're doing it for their lives - that's the most important thing.
"When we start games [against big clubs], our fans clap a striker making a tackle. It's always been in our culture.
"You have to have that work ethic and we're scouting the market, trying to find the right ones."