De Gea could have played his last match for United after he hobbled off with a hamstring injury in the 74th minute of their 1-1 draw against Arsenal on Sunday.
Valdes came on to make his United debut in De Gea's place and is likely to retain the gloves for the final match of the season at Hull next weekend.
Having spent 12 years in Barcelona's first team battling against Real Madrid, Valdes knows all about the strengths of last year's Champions League winners.
But although he regards Madrid as one of the biggest clubs in the world, Valdes is in the dark over whether his compatriot will be heading to the Spanish capital this summer.
"Madrid, Barcelona, and Manchester United are three of the biggest clubs in the world," the United goalkeeper told Press Association Sport.
"Everyone in the world wants to play for these clubs
It's a question you would have to put to David
I cannot answer for him
I don't know (if he is leaving)."
Valdes picked the ball out of the net eight minutes into his United debut after Theo Walcott's cross flicked off Tyler Blackett and ended up in the goal.
It was an unfortunate way to return to action after 14 months on the sidelines.
Following his comeback from a serious knee injury, Van Gaal - the man who handed De Gea his debut at Barcelona - offered the goalkeeper a chance to train with United and in January, he signed an 18-month contract.
Until Sunday, Valdes' only appearance in a United shirt came for the under-21 side
The 33-year-old insists he has enjoyed his time at the club since signing in January, though.
"It's been good, all good," the three-time Champions League winner said.
"My team-mates have treated me well here
The coach has treated me well
The truth is that it has all been good.
"I am very happy to be here
I have one more year on my contract here
Let's see what happens."
Despite sustained chants of "David de Gea, we want you to stay", the United fans gave their new goalkeeper a good welcome when he took to the field on Sunday.
The Spaniard appreciated the gesture and is keen to repay the supporters after what was an unexpected debut.
"It felt very good
It all went very quick
David asked to come off very quickly and the next thing I knew I was on," Valdes said.
"It was a great feeling to be on the pitch at Old Trafford, the fans welcomed me very well and for that reason I am happy.
"I wasn't happy with the result because it wasn't a good result for us
It was a lucky goal for them.
"But I'm happy I made my debut
To come on and be welcomed like that is incredible for a footballer
They are an incredible group of fans."
The result means Arsenal are all but guaranteed to finish in third place, thus granting them direct entry into the Champions League.
United will have to go through a potentially tricky play-off to make the group stages
Potential opponents include Monaco, Lazio or Sporting Lisbon.
After last year's dismal showing under David Moyes, Champions League qualification was Van Gaal's target this year, but Ashley Young thinks United will be challenging for the title next term.
"As a club, we always want to finish as high as possible," the winger told MUTV.
"It looks like we'll finish fourth and we have to go into the last game and get the three points.
"Then we'll dust ourselves down in the summer and come back again
We want to bring Champions League nights back to Old Trafford - they're fantastic.
"We'll come back stronger next year and we'll look to win the league."
Source : PA
Source: PA