Former United captain Gary Neville criticised the pair for their actions, despite the club granting them permission.
Amorim told Sky Sports: "The first thing is it was five days off. The second thing is they received information of five days off and they are big boys, they have kids so they decide what to do.
"The main question here is the club have to set the standard and manage that. it's my decision if they can have five days as a coach, or three days. Or is three days to rest, you cannot fly. This is something the club has to decide.
"You cannot put this on the players. they told them they have five days off so they can fly anywhere. Nobody in the club said they cannot fly. They have to live their lives because they are grown men and they have to decide these things. Us as a club have to change in these standards.'
"In your time you had a great leadership at the club, very strong and the culture was already here when you start. So it was a long time with the same identity, the same way of seeing things and you felt that even if you do that, your team-mates will talk to you.
"Now it's a different point, you have to acknowledge that. This must be started in the club, with us. We are responsible in that area. We cannot in this time put that of Rash or Casa. They receive the information five days off, do what you like. We as a club have to set better standards and we will try to do that."