Van Gaal wants a relaxed Rooney
Van Gaal takes charge of his first competitive match as United boss on Saturday when Swansea come to Old Trafford.
It will also be the first time Rooney has led the team out in his new role as United's full-time captain.
Rooney was announced as the club's skipper on Tuesday night, following the 2-1 win over Valencia.
Rooney also wore the armband in the friendly victory, but he put in a sub-par performance on the night, missing a penalty in the first half.
Van Gaal believes Rooney has the potential to be an excellent United captain, but the Dutchman thinks the striker must not be overawed by the responsibility.
"I said to him (after the Valencia game): 'Maybe it was too much for you as a captain'," Van Gaal told a press conference.
"But I said to him - 'let it fly', because when we were (on tour) in the US, he scored a lot of goals, and made a great amount of assists so I wanted to say that."
Given that Van Gaal enjoys a good relationship with Robin van Persie, who skippered Holland under the United boss at the World Cup, Rooney's appointment as captain came as something of a surprise.
Van Persie even missed out on the vice-captaincy, which was awarded to long-serving midfielder Darren Fletcher, who had an excellent pre-season tour.
It was Rooney's work-rate and application in pre-season that won him the honour of following in the footsteps of club greats such as Sir Bobby Charlton, Bryan Robson and Eric Cantona.
But before he handed Rooney the armband, Van Gaal reminded the player - who has had his moments of ill-discipline in the past - he must be a fine ambassador for the club away from the field of play, as well as on it.
"His attitude, of course," Van Gaal said when asked why he selected Rooney.
"That is in the pitch and on the training ground and I have explained to him also outside the pitch is also very important for me.
"And he accepted the responsibility and I like very much how he did train and also how he performed in the game and how he performed to his fellow players."
There was no sign Van Gaal was suffering from any pre-match nerves on Friday, even though he admitted his preparations have been disrupted by a series of injuries.
Antonio Valencia, Luke Shaw, Jonny Evans, Danny Welbeck, and back-up goalkeeper Sam Johnstone are unavailable.
Van Persie is still short of match fitness following his late return from the World Cup and it is unlikely Adnan Januzaj and Marouane Fellaini will start against Swansea for the same reason.
But after six straight wins in pre-season - against teams who have amassed 35 major European trophies between them - Van Gaal is not daunted by the prospect of trying to bring the glory days back to Old Trafford following last year's seventh-place finish under David Moyes.
"I am very confident. I am not nervous - don't you see it?" said Van Gaal, his booming voice reverberating around an "unbelievable" new room at United's training ground in Carrington.
"I am not worried. We have beaten them all until now."
Van Gaal lamented the loss of Luke Shaw, the £27million left-back who is out for a month with a hamstring problem.
The former Barcelona manager listed six reasons why Shaw suffered the injury. Three of them were related to United's five-match 13,500-mile pre-season trek around America.
United are hopeful of reaching an agreement with Sporting Lisbon over the transfer of Argentina defender Marcos Rojo, who has a 30 million euro (£24m) buy-out clause in his contract.
For now, Van Gaal could deploy either Ashley Young, Tyler Blackett or Reece James at left-back.
Neither Blackett nor James have played a competitive game for United, but their 63-year-old manager is not afraid of handing them their debut on Saturday.
"I don't think so," Van Gaal said when it was suggested United are short of left-backs.
"We have played in the USA with Ashley Young and Reece James in that position, so that position is very well-covered."
For the fans who became disenchanted with watching United under Moyes last year, Van Gaal had one final message.
"The most important thing is the fans shall understand how we play and the way of playing will give pleasure to them," Van Gaal said.
His English may not be polished to perfection, but the message coming through was clear enough - Van Gaal is on a mission to put smiles back on faces at Old Trafford this year, and he would love to start with an emphatic win over Swansea.