United travel to Camp Nou looking to pull off a comeback after going down 1-0 in the first leg at Old Trafford.
Messi may have drawn a blank in Manchester, but the Argentine star has scored 43 times in 41 games in all competitions this season and is the Champions League's top scorer.
Well rested after being left out of the Catalan side's 0-0 draw with Huesca on Saturday, Messi will be raring to go as Barca look to keep their hopes of winning a treble of trophies alive.
But De Gea is feeling up to the challenge, according to his coach, having put in extra work to get ready for the five-time Ballon d'Or winner.
"Those who know David well can tell you one of his virtues is his ability to block everything out, he is very strong mentally," Alvarez told Cadena SER.
"We do something different when facing Messi. With De Gea we have worked on some more specific work, there are some extras compared to other games with having Leo up front.
"I wouldn't lie if I said we do a super special preparation. I worked in La Liga for many years with David as well and before with other keepers. I can't deny that there is a particularity regarding Leo. In the end, you try to offer tools and data to the keeper so that it can help him.
"In this game we have a lot of respect for the opponents and their players, but no fear. We have been able to beat important rivals like Barcelona, so we are capable of winning against any team."
United overturned a two-goal deficit to eliminate Paris Saint-Germain from the last round of the competition, giving them a boost of confidence ahead of the crucial clash in Catalunya.
"The most important thing is to know that the Champions League is a different competition than the Premier League," Alvarez added. "The way we eliminated PSG is a reinforcement for us. It is the most important game of the season."