Athletic Bilbao striker Fernando Llorente says Wayne Rooney will be Manchester United's greatest threat when the two sides square off in the Europa League last 16 on Thursday night.
The England international has netted 25 goals for United this season, and has often proved himself to be the difference at crunch moments in the season for the reigning Premier League champions.
And Llorente, La Liga's fifth top scorer, insists the threat of Rooney is one of the reasons behind United's favourites tag heading into the first match between the two for 55 years.
"Manchester are obviously the favourites. They always play Champions League and compete with teams like Barcelona and Real Madrid," Llorente told the Europa Press Association.
"They are one of the big teams in Europe and we're novices.
"Rooney is a real killer. He has a great mentality and is a great inspiration for his team-mates.
"I expect a very exciting game. We are really looking forward to the game, and will do our utmost to get a good result. We have been doing quite well and might cause an upset."
United struggled through their last 32 clash with Ajax, losing the second leg 2-1 to the Dutch champions following a comfortable two-goal victory at the Amsterdam Arena.
Rooney was absent from the United side who went within one goal of crashing out of their second European competition of the the season, following their Champions League exit last December.
The 26-year-old returned to the United fold after a three-game absence with the opening goal at White Hart Lane, as Tottenham Hotspur were brushed aside allowing Sir Alex Ferguson's men to close the gap at the top to two points from leaders Manchester City.