The forward gave Arsenal the lead with a clever near-post flick before he smashed another into the top corner during Sunday's convincing win at Emirates Stadium.
They were Alexis' fifth and sixth goals in the week, and Cech believes the 26-year-old is getting back to his best after recovering from a busy summer with Chile.
You can see that he feels confident and with performances like that, it’s great for the teamPetr Cech
“It’s always hard for the players when you have the Copa America,” Cech told Arsenal Player. “He won it, he was there for a really long time, didn’t have much break and then came back and needed to start again.
“You could see that everybody from the Copa America had a slow start, in other teams and competitions as well. We’re pleased that he’s back with his energy, with his enthusiasm and you can see that he feels confident and with performances like that, it’s great for the team.
“We put in the right intensity and the first goal helped,” he added. "Once you go one up, you can get the game under control. What was good was that we searched for the second and third goals and they came pretty quickly. We managed to get ourselves the advantage and I think we did remarkably well to control the game afterwards.”
Sunday’s win was Cech’s fourth Premier League clean sheet in Arsenal colours and he was delighted with how well the team defended.
“Every clean sheet counts,” he said. “There was an important save right at the end of the first half, which I’m pleased with. The whole team defended very well as a unit against Manchester United and we deserved the clean sheet.”Copyright 2016 The Arsenal Football Club plc. Permission to use quotations from this article is granted subject to appropriate credit being given to www.arsenal.com as the source