The former Atletico Madrid man has come in for heavy criticism in the past couple of weeks. De Gea was blamed by Gary Neville, among others, for a stoppage-time Tottenham equaliser a fortnight ago, prompting manager Sir Alex Ferguson to hit out at the opinions of "idiots".
But De Gea produced a series of excellent saves at Fulham on Saturday as United eked out a crucial 1-0 win, and Rooney said: "David has made some great saves over the last few weeks. I know he got a little bit of criticism after the Tottenham game but it wasn't the biggest mistake I've ever seen. We're all behind him. He's a great player for the future as well."
There are still issues to be addressed about De Gea's future.
Real Madrid coach Jose Mourinho is believed to be a fan and there have been suggestions the Primera Division champions will attempt to take Spain's Olympic keeper home in the summer, even if there is a change of manager at the Bernabeu.
And the certainty Jack Butland will join Stoke in the summer gives credence to speculation that Asmir Begovic will make the short journey north from the Britannia Stadium to Old Trafford at the same time.
Even Ferguson's post-match admission on Saturday that goalkeeping coach Eric Steele has been urging De Gea to enjoy his time at Old Trafford is an indicator all is not well with a player who cost £18million just 18 months ago.
But, for now at least, United are circling the wagons around someone, for good or bad, they will have to rely on for the remainder of the campaign.
"People underestimate the reflexes he's got," defender Jonny Evans said.
"We are used to seeing him do that. His shot-stopping ability is very good and his save from (John Arne) Riise was excellent."