Simon Grayson believes that the potential arrival of goalkeeper Sam Johnstone will only be confirmed should David de Gea leave Manchester United.
De Gea has been heavily linked with a move to Real Madrid and his move could prevent Johnstone rejoining Preston.
"United are waiting to find out what is happening with David de Gea," Grayson told Lancashire Evening Post.
"Ben Amos has left to go to Bolton and that has left them a bit short of keepers. They will want to carry three keepers in their 25-man squad this season.
"If De Gea leaves and they don't bring anyone in, Sam might be third choice which rules out bringing him here."
Johnstone made 22 appearances on loan at Preston last season.
