This makes no sense, you are confusing fitness from an athletic standpoint and fitness from a medical standpoint. Both players will lack sharpness and feel for the game when they missed several months but in terms of fitness, you get in shape in a matter of weeks, Pogba has been training since February, he has been working on his fitness for almost 5 months while Rashford was unable to train and forced to rest for the better part of these 5 months. In terms of fitness they are most likely at the same level and a good one, the question is whether Rashford is fully healed because unlike Pogba, June was the optimistic return date.
There is no argument to make where Rashford playing 90 minutes makes more sense than Pogba playing 90 minutes, unless something happened during the week.