Sevilla welcome United to the Estadio Ramon Sanchez Pizjuan for the first leg of their last-16 showdown on Wednesday night, with Sanchez due to make his first Champions League appearance of the season.
Montella revealed that he has been aware of Sanchez's quality since he moved to Europe with Udinese, and warned his players that they face a difficult task in trying to stifle the Chilean.
"He's a fantastic player, I saw him play in Italy. He has improved so much as his career has progressed, he's very fast, he leads counter-attacks, he's very strong," Montella told reporters at his pre-match press conference.
"How do we stop him? I don't know, we'll have to block him in some way, maybe tie him up with a rope."
Sanchez has scored one goal and registered one assist in his five appearances for United so far.