The legendary Dutch coach was drafted in to replace David Moyes after the Red Devils endured one of the worst title defence in their history during the 2013-2014 season.
Van Gaal spent two seasons trying to bring back success at Old Trafford but was swiftly replaced by his former protégé Jose Mourinho, 24 hours after guiding Manchester United to lift their first FA Cup since 2004.
Rooney has admitted he was stupefied when the board relieved the former Ajax, Barcelona and Bayern Munich boss of his duties after just two years at the club and believes United could have won a lot under him.
"I was devastated when Louis was sacked. For me, it was an absolute joy to work with him," the 34-year-old is quoted by The Mirror in an extract from a new book, LVG – The Manager and the Total Person.
"We should have kept him for a third season. We would have been so much stronger.”
"I felt things were improving and players started to understand his vision. In those two years I learned more than under any other manager.”
"This is why I will be forever grateful to him. Not just for making me captain, but also for all the trust and belief he had in me.”
"We didn’t have the best team in the league anyway, but we could not afford to have 12 players injured.”
"Our best XI was good enough to play in the top four, but once we got injuries we got in trouble because we did not have the same quality in the squad as in the years before.”
"At the time it was good for me because I had decided that I wanted to become a manager. And working with Louis in that way was priceless in my opinion, because I could learn so much from him. I could not have wished for a better example.”
