I suggested Perez only as an example (ie, a big name club president who is active in transfer dealings in comparison to an agent) now suddenly people think it's him!
It's more likely to be an agent than a president for a game like West Brom, which would attract considerable attention but not have the guise of watching a big/significant game.
Mendes seems a good guess, or Wagner which tolm hinted to on Friday, but it could literally be any agent or even a family member (in the case of say, Kylian Mbappe)