I actually think that Southgate is trying to protect Grealish from injury. If you noticed most of the team last night were 1-11 in their shirt numbers, so I think this an indicator of who he sees as his first eleven. Given that Grealish cannot train every day, he is trying to use him where he can be most effective. I thought Saka was excellent, and both fullbacks were shocking. Given that he has three decent centre halves i would rather he played them and bring in two wing backs.