Having established himself as one of Manchester United’s most reliable performers over the past few seasons and one of the best goalkeepers in the world, De Gea produced a stunning save in the last 10 minutes of the 1-0 win over Young Boys in the Champions League group stages at Old Trafford on Tuesday.
Marouane Fellaini grabbed a last-gasp goal to settle the game and send United through to the knockout stages of the competition with a game to spare.
De Gea, meanwhile, has faced a lot of speculation over his future as his contract expires in 2019, although United have the option to trigger a further 12-month option.
Juventus and Paris Saint-Germain have been closely monitoring the Spaniard.
But Mourinho is adamant that United will do all they can to keep suitors at bay.
“Without David de Gea’s save, no winning goal. We qualified. We did it,” Mourinho said.
“He is the best goalkeeper in the world.
“If our ambitions are to be the best goalkeeper in the world, we have to keep him.”
On United’s performance, Mourinho continued: “Relief. We didn’t play to be in trouble until the last minute.
“I was frustrated we couldn’t score.
“My players were very tired at the end.”
Mourinho also reckons United face a tough task trying to reach the quarter-finals of Europe’s elite competition, adding: “Realistically to be in the last 16 is an achievement, not a big one or a fantastic one, but an achievement.
“There are better teams than us. Teams with more quality and realistic ambitions.
“When a team arrives in the quarter-finals, anything can happen.”
United are second in Group H with 10 points and have qualified for the knockout stages with a game to spare.