Mourinho plans to start 20-year-old 'keeper Joel Castro Pereira in United's final Premier League match of the season at home to Crystal Palace on Sunday, while Sergio Romero will start the Europa League final against Ajax on Wednesday.
De Gea has been relentlessly linked with a move to Real Madrid in the forthcoming summer transfer window in recent weeks.
Asked when De Gea might be seen in a United shirt again, Mourinho said: "The first match against LA Galaxy in pre-season in Los Angeles".
"I have been here seven years and I never saw any detail trying to care about the English teams involved in European competitions".
"But David is top in the world and obviously we want to keep the top in the world".
Manchester United manager Jose Mourinho has launched a new attack on the Premier League by accusing them of hindering his team's bid for Europa League success.
Mourinho also said some players gave him food for thought ahead of the Ajax game. "Tells (Wilfried) Zaha to go slow, leaves (Christian) Benteke at home; I hope he goes soft on us".
Despite the hard-fought draw, in which Southampton would have won but for the brilliance of United's stand-in goalkeeper Sergio Romero, manager Claude Puel remains under pressure. "We need two top goalkeepers and a top young goalkeeper and this is what we have".
Having stretched his winless league run to five games as the unimpressive run-in left United certain of finishing sixth, Mourinho can only hope his injury-hit side fire on all cylinders in Stockholm. "They showed me that they want to be in the final, they want to be an option".
"We needed to be clinical perhaps, but we played good football and had chances to win the game".
"Any teams have a debrief at the end of the season and this is normal but the end of the season is after the last game", said Puel.
"It's a shame I think because in the first half we had good play without quality or intensity and the second half it was better with possibilities and opportunities".
"I think we deserved to score".