The Gunners lost 1-0 in the first leg at Old Trafford.
“We know it will be a big game for us. It’s our biggest game of the season, also the biggest game the Emirates will stage,” he told skysports.com.
“We will need the fans to be behind us for 90 minutes, even for 120 minutes!
“It will be hard because they are the European champions, and they are back on song following their blip.
“We believe we can do it. We are ready. It’s a massive game.”