The 39-year-old arrived last month to succeed Erik ten Hag and has lifted the mood at Old Trafford with his charisma. However, O'Shea, who played for United from 1999-2011, said the players had to be convinced of the manager's capabilities.
"In terms of a personality, I really like [Amorim]," O'Shea, the current assistant manager to the Irish national team, told a small group of reporters recently. "[But] it has to not be everything. You're not going to get away with just being able to talk well in the media.
"You have to have the respect of the players when you go in and try and explain how you're going to approach a certain team, why you're going to do this today, why we were doing this last week and not doing it this week.
Ruben Amorim
Ruben Amorim
"We want to win" Manchester United manager Ruben Amorim reacts after loss
"You're getting that balanced approach right and getting the belief into the group again, and you're trying to do that when you're changing four or five players a game because there's fitness issues at the minute with the team.
"You're hoping that certain players stand up quickly and be counted. That's what's needed especially," O'Shea added, speaking in his role as ambassador for the club's 'United We Play' initiative.
With losses to Arsenal and Nottingham Forest in their most recent Premier League games, United will be looking to turn things around on Thursday when they play Czech team Viktoria Plzen in the Europa League.
The 43-year-old O'Shea, who won five Premier League titles and a Champions League trophy with United, said former Sporting coach Amorim will be keen to start shaping his squad more to his liking as early as the January transfer window.
"That freshness and energy that the manager will want in his team because of how he wants to play, that's a key part, because obviously there's a new energy and freshness from his staff, but the players will have to respond to that quickly because of the amount of games in December, it's crazy," O'Shea said.
"So I'm sure Ruben will be thinking about the January transfer window, as much as he's not saying he is, and I'm sure in the background the club will be doing all they can to help him in whatever way they can possibly, to adjust to certain positions in the system that he wants to play."