Harry Maguire did not take part in training with Leicester City on Monday, after phoning in sick.
Sky Sports News understands Maguire has told Leicester he wants to leave before the transfer window shuts on August 8 and he remains Ole Gunnar Solskjaer's number one target at Old Trafford.
United's executive vice-chairman Ed Woodward remains in talks with Leicester about a deal, after Solskjaer made it clear he wanted to sign Maguire as soon as possible.
It is understood United would have to pay Leicester £90m for Maguire, further underlining the Foxes stance that they feel under no pressure to sell the player.
Woodward decided to not travel out for United's pre-season tour of Australia and the Far East to work on the ongoing negotiations.
According to Sky sources, it is now up to Woodward to get the deal done.
Speaking to Sky Sports News from Leicester's training ground on Monday, where the players were posing in front of green screens for the annual pre-season photo-shoot, club captain Wes Morgan was full of praise for his centre-back partner's ability in possession.
"Back in the day it was not about centre-backs getting the ball and playing from the back. Nowadays it is clear to see that centre-backs have the most touches on the ball," he said.
"Harry has taken that to another dimension. He can get it in tight positions and take the ball through players. He can pass, but first and foremost he is a good defender.
"I am just glad he is playing for Leicester."
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Fantasy Football: Maguire the man?
Whether Harry Maguire stays with Brendan Rodgers at Leicester or makes the move to Old Trafford is yet to be seen, but you can add him to your Sky Sports Fantasy Football team for just £8.9m. The centre-half has already been selected by 18 per cent of Fantasy Football bosses.