The Foxes boss has so far been without two of his influential players throughout the entire pre-season because of their involvement in England's World Cup squad in Russia this summer.
As Gareth Southgate's side made it all the way to the third-placed play-off on the final weekend, it means they won't be back in training with Leicester until Monday.
The Foxes open their campaign against United at Old Trafford next Friday night, August 10, and Puel does not feel central defender Maguire or striker Vardy will be ready after just four days back.
"Of course Jamie and Harry are valuable players for us, but we need to protect them," Puel told reporters after the 1-1 pre-season draw against Valencia on Wednesday.
"If they are available to play in their minds, it is a good thing because this shows their desire to come back and fight with the team, but we have to take a protective decision from us."