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".
There were plenty who felt the 22-year-old was also responsible for Southampton's early goal on Wednesday, even though Michael Carrick admitted it was his fault.
But De Gea produced a series of excellent saves at Fulham on Saturday as United eked out a crucial 1-0 win.
And match-winner Rooney was quick to highlight the Spaniard's contribution.
"David has made some great saves over the last few weeks," the England frontman said.
"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."
That effort - and another to deny Bryan Ruiz - were the highlights for De Gea during a thrilling opening that seemed certain to yield plenty of goals.
United struck the woodwork on three occasions during that spell, including a curling shot from Rooney.
But a 10-minute break caused by a floodlight failure halted the momentum of both sides and the match was heading for United's first goalless stalemate in an incredible 97 games until Fulham's defence failed to deal with a long Evans punt downfield 11 minutes from time.
Rooney was onto it in a flash, finishing smartly for his 10th goal in as many games to confirm a victory that means United have now collected 35 points from 39 since their last defeat at Norwich on November 17.
And while Ferguson will not allow minds to wander with a record 20th title to win, Rooney accepted it felt very much like a memorable Cristiano Ronaldo goal on the same ground in 2007 that was pivotal in the first of a championship hat-trick.
"I fancied myself to score," Rooney said. "It was a little bit similar to Cristiano's goal.
"After that one we went on to win the league and hopefully that can happen after today."
With the blockbuster Champions League encounter against Real Madrid looming, progress may not be quite as smooth as it has been for United, so their present points advantage over Manchester City may be needed.
However, Fulham manager Martin Jol is not constrained by protocol.
And he has no problem confirming the general belief that United will end the season by being crowned champions.
"I think they will win it," he said. "Rooney was always a problem for us. At Old Trafford last week he was unbelievable.
"This time we kept him quiet. But he scored a goal out of nothing. That is him."