The Red Devils flew out last weekend and they're taking on Perth Glory tomorrow.
Maguire would become the world's most expensive defender, beating Virgil van Dijk's £75m fee when he moved from Southampton to Liverpool.
The Foxes want £90m for the England centre-back - but there is hope a deal can still be done.
But United have been put off midfielder Longstaff.
United will pay £20m and thought a deal would be done. Inter's hopes of being able to meet United's £75m valuation are still likely to hinge on them offloading Argentina striker Mauro Icardi, who is attracting interest from Juventus.
Longstaff, 21, is desperate to move to Old Trafford after his representatives shelved contract negotiations with Newcastle in March, but has been left in the dark as talks continue.
That said, he is a local boy who may realise he needs a full season in the top flight to prove himself as well as the fact Manchester United have wrecked many a players' reputation in recent times. You have to have the right players at the right price'.
Romelu Lukaku and Anthony Martial missed the session but United insisted that both players trained in the gym even though further talks took place with Inter Milan in London on Wednesday over a deal for Lukaku.
The Italians' manager Antonio Conte wants the Belgian, 26, to be his main man up front.