Andre Onana is still trying to make it back from injury in time to face Arsenal, despite initial recovery timelines ruling him out until after the match.

Laurie Whitwell reports for The Athletic that though Andre Onana is currently working on his recovery from a hamstring injury, he’s planning to fly to Chicago with the Manchester United squad to be around the camp for pre-season.
Whitwell adds that Onana is aiming to be back in time for the game against Arsenal on the opening weekend of the season.
United’s pursuit of a new goalkeeper is also believed to be unrelated to Onana’s injury, so if they do make a signing it’s evidently not just to cover for their Premier League opener.

Initial reports from Mike McGrath of The Telegraph and Alex Crook of talkSPORT had put Onana’s injury timeline at between six and eight weeks out.
That would mean a return between August 23rd and September 6th, with Arsenal travelling to Old Trafford to face Manchester United on August 17th. But those were early estimates, and things can always change.
As it stands, Altay Bayindir would be the most likely player to take Onana’s place in goal against Arsenal on the opening day of the season. United also have Tom Heaton and Radek Vitek as alternative options, depending on how pre-season goes.

If he’s unavailable, Onana would be United’s second player to be ruled out of the Arsenal game, with Lisandro Martinez also still sidelined.
Mikel Arteta’s side will be without Gabriel Jesus for the encounter, but the hope is that everyone else will be fit by August.