The Sporting CP coach's move to United was confirmed on Friday.
Louro was in Jose Mourinho's backroom team at United and told O Jogo: "Understanding (Amorim's decision), of course. In fact, it seems to me that there is no way not to understand. It is an opportunity to train a big club worldwide.
"Sporting is also a great club, but, in a cold and honest analysis, you cannot compare the size of the two. This opportunity has arisen for Ruben, who will certainly take the opportunity to emancipate himself as a coach at the highest level in Europe.
"Beyond the less good moment the team is going through, he will enter a club where losing a game becomes a tremendous problem. There, you always have to win, there is no other way. However, we have to take into account that nowadays there are teams with great arguments in the Premier League, such as Manchester City, Liverpool. That's no excuse, because United have quality and they're still a giant. But the results have not been the best, fact.
"For all I have seen of Ruben Amorim in Braga and Sporting, I believe he will be able to replicate the same success in Manchester. Tactically he is very strong, as we have seen, and he is able to create a great relationship with the players. Therefore, I believe in his success and hope that it will be so."