Van Gaal was axed in favour of Jose Mourinho shortly after winning the FA Cup, with United having finished outside of the top four in the Premier League.
Under Mourinho, United finished sixth in the final 2016-2017 Premier League standings but won both the Carabao Cup and Europa League.
Yet Rooney, who was United's captain at the time, insists the club should have given Van Gaal another year to prove he could be a success.
"I was devastated when Louis was sacked," Rooney said in the book 'LVG - The Manager and the Total Person'.
“For me, it was an absolute joy to work with him.
“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."