The Spanish goalkeeper was offered a new contact earlier this year but the deal remains unsigned.
De Gea's current deal expires at the end of June and United fear that they could lose him for nothing next summer.
And if the former Atletico Madrid (opens in new tab) shot-stopper has not penned fresh terms by January, United may opt to cash in midway through the campaign.
The Red Devils are therefore considering replacements, with Jan Oblak a long-term target.
It could be difficult to prise the Slovenian away from Atletico, however, so Dinamo Zagreb's Dominik Livakovic has emerged as a cheaper alternative.
Why the English midfield stereotype is dead – as Jordan Henderson proves (opens in new tab)
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Greg Lea is a freelance football journalist who's filled in wherever FourFourTwo needs him since 2014. He became a Crystal Palace fan after watching a 1-0 loss to Port Vale in 1998, and once got on the scoresheet in a primary school game against Wilfried Zaha's Whitehorse Manor (an own goal in an 8-0 defeat).