Ronaldo, now starring with Al-Nassr, left United after his second spell in 2022 for the Saudi Pro League.
The 39 year-old was speaking at the Globe Soccer Awards in Dubai after being named Middle East Player of the Year.
"He (Amorim) did a fantastic job in Portugal with my Sporting," said Ronaldo.
"But the Premier League is a different beast, the most competitive league in the world. I knew that it would be tough and they will continue the storm.
"But the storm will finish and the sun will rise. Things crossed, it will be good with him and I hope the best for Manchester United because it is a club I still love."
Ronaldo added: "I will continue to say, the problem is not the coaches.
"It's like the aquarium and you have the fish inside and it's sick, and you take him out and fix the problem.
"If you put it back in the aquarium it will be sick again. This is the problem of Manchester United. It is the same."