Iraola's side are sixth in the table, seven places ahead of United but just three points clear of their opponents.
Brought in after Erik ten Hag was sacked as United boss, Amorim's stint began with a draw against Ipswich.
United have since won four games and lost three, with last weekend's derby defeat of Manchester City followed by a League Cup loss against Tottenham.
"I think they are different," said Iraola, whose side won the equivalent game 3-0 last season.
"I think they play in a different way, different system, different approach to the games," he also told a pre-match press conference on Friday.
"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 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."
United, however, fell 3-0 behind against Spurs before losing 4-3.
"I think probably they deserved more," said Iraola. "A lot of goals but I think they had very good moments."
United, however, have looked vulnerable at set-pieces.
"It's true that they've conceded a good amount of goals from set-plays but I think they have very good defenders and a lot of the goals are very good execution from the opposition," said Iraola.
"I hope we can, as we do every week, prepare well also for set-plays, try to be successful. We are not the tallest of teams. More the opposite, we are quite a short team, but I think we are having some success in our offensive set-plays."
Bournemouth's three-game winning run was ended by a draw with West Ham last time out.
"We have to go step by step," said Iraola. "We still need more points. Twenty-five is a good amount but, if you see the standings, every team is quite together so we have to continue trying to add points."
