However, the Real Madrid star and former Manchester United forward feels that the 63-year-old should be given time to bring success to the club, even if he fails to win the title this season.
Ronaldo told reporters: "Van Gaal is a fantastic coach and I am certain he will make United champions again – if not this year then very soon after that.
"Sir Alex Ferguson said that Van Gaal will need time - and he is right. Because, in the beginning at United, even Sir Alex didn't win anything for a few years.
"To get a good squad and a good team takes time, so lets give Van Gaal the time he needs."
Ronaldo played an integral part in Real's El Clasico win over Barcelona on Saturday as his side inflicted a first defeat of the season on Luis Enrique's men.