Although their domestic form has improved, with seven points and 10 goals in three games since they were beaten at Old Trafford a fortnight ago, Sociedad will start rank outsiders.
They are yet to collect a point in the present European campaign and know another defeat could even end their hopes of nabbing a Europa League consolation prize by finishing third.
Arrasate is remaining upbeat.
He may think United are also emerging with more confidence from the transition phase that inevitably accompanied David Moyes' arrival as Sir Alex Ferguson's replacement.
But that does not mean he feels Sociedad have any need to worry.
"A lot has been said about this transition period but Manchester United are still very powerful," Arrasate said.
"They have been struggling a bit in the Premier League but they are coming back now. They are the current champions.
"It is the same squad. They have a lot of potential and they are still a great team.
"But we have played better in our last two games. And we can play better than we did at Old Trafford.
"We have nothing to fear and if we play our cards right, we can give them a scare."
Sociedad will certainly be paying plenty of attention to Adnan Januzaj, should the 18-year-old make his European debut in the Anoeta.
Januzaj has made a big impression in Premier League combat this term, underlining why there has been such a scramble to claim his international commitment.
"I didn't know a lot about him before but we have studied him now," Arrasate said.
"He is a young player who is very decisive at times.
"He is very quick and has a great future. But there is a lot of competition for places at Manchester United, so we will see."