According to The Telegraph’s Mike McGrath
Onana picked up the injury in training after he and other
teammates returned for preseason which began on July 7.
“Manchester United have been dealt a massive blow with
André Onana ruled out of pre-season tour with a hamstring
injury. Ruben Amorim is now weighing up whether to buy
another goalkeeper before the Premier League season gets
under way.”
“Onana, 29, has suffered the problem this week as players
returned to United’s training base at Carrington, with early
assessment fearing he will miss six to eight weeks of action,
pushing him to the brink of making the curtain-raiser against
Arsenal on Sunday August 17