The Old Trafford clash with the Gunners is creeping ever closer, taking place on Sunday 17 August, with a 16:30 BST kick-off, as Ruben Amorim's side look to begin the campaign in positive fashion.
Onana travelled to the United States with the rest of the squad but worked on his fitness as he bids to return from injury.
The Cameroon international did not feature in the Premier League Summer Series, with Altay Bayindir and Tom Heaton keeping goal.
Bayindir played against West Ham United and Everton, while Heaton started the 4-1 victory over Bournemouth.
Speaking to NBC Sports in Atlanta, Onana issued an update on his fitness and stressed the importance of getting off to a good start against Mikel Arteta's side.