Monreal: When Sanchez is fit, the level of the team is higher.
After a below-par performance in their midweek defeat to Olympiacos, Arsene Wenger's side responded with a vital win over Manchester United.
Arsenal ran riot over Louis van Gaal's men on Sunday at the Emirates as Alexis Sanchez's double and Mesut Ozil's cool finish gave them a 3-0 home win.
And, Nacho Monreal said a strong start was important in their victory as he heaped praise on Chile international Sanchez.
"Last year he showed what an important player he is for the team,” Monreal said as Arsenal.com quoted. “He scored 25 goals in all competitions and this season he has started really well.
“In the first games, he couldn’t score goals but now he’s scoring goals. We need Alexis to score more goals and we are happy for him because he’s a really important player for us.
“When he’s fit, the level of the team is higher, so we are really happy.
“We wanted to start stronger because in the last game, we played at home in the Champions League and we lost, so the memory wasn’t good enough for us,” Monreal added. “We knew that we were going to play against a strong team so we needed to start really strongly.
“I think we did - our first 15 minutes were amazing. We pressed high and I think we did everything amazingly.
“For this reason, after 20 minutes we were winning 3-0. We are really happy about our performance.”