Iraola says there's been a distinct switch since Amorim replaced former manager Erik ten Hag.
Ahead of Sunday's clash, the Cherries manager said: "I think they are different.
"I think they play in a different way, different system, different approach to the games. I think they keep the games more in their control. They've raised the possession they have on the ball and they have really good patterns.
"If we allow them to dictate the game and to be the team taking the decisions on the ball, it's going to be very difficult because I think they are playing very well.
"They are conceding very few chances, very few goals, they have very good defensive stats. They already had this with (Erik) ten Hag this season because they've signed also new defenders, good defenders, and they've improved from past seasons defensively overall and I think they are continuing now."
On United's 4-3 Carabao Cup quarterfinal defeat at Tottenham, Iraola also said: "I think probably they deserved more.
"A lot of goals but I think they had very good moments. Playing against Tottenham away, it's always a difficult place to go and I think overall in the games they've been composed.
"Even when they are losing, when they want to risk a little bit more or open the game a bit more like yesterday second half, they can do it. I see they manage very well the moments of the game. I think it's going to be difficult for us."