The story on the Sky Sports website does have the sub-heading "Robertson could miss start of 2019/20 Premier League season."
However the same story quotes the Scotland manager as saying ""He tried everything he could [to be ready for today], he did the training session last night, but he woke up and still felt it so no good. And it is a great chance for Greg Taylor."
Which doesn't make it sound as if the injury is that serious.