Erik ten Hag’s side finally got their season up and running on Monday by beating bitter rivals Liverpool, although that will count for little should United not back it up on the south coast in Saturday’s early Premier League kick-off.
Hasenhuttl, whose side came from behind to draw 2-2 with Leeds in their opening home match before winning 2-1 at Leicester last week, is unsure how Ten Hag will approach the visit to the south coast.
"I don’t know if this was the reason why they won the (Liverpool) game," he said of the omissions of Maguire and Ronaldo.
"Sometimes changes are because of tactical thoughts you have and that makes it always difficult to be sure what squad we are facing.
"Even if Casemiro is playing it is not 100 per cent clear in what way it is a different squad.
"We are definitely facing a very strong team here and in the end it’s up to us to keep on going in the way we want to go.
"We have shown in the last two games that we are competitive and this is the next big challenge and we want to show ourselves how good we are."
Additional reporting from PA