1: By having a smaller economy as a result of Brexit the economy can afford fewer items.
2:By restricting migration as a result of Brexit you end up with fewer providers. So no Ulloa, Calderon, Bruno, Knockaert, but more Leon Best's and Chris O'Grady's (I'm assuming here that if the government is serious about reducing migration by 60-80% that non-international footballers are not top of the list of essential employees.
We will have control of immigration - i.e. the UK set the rules and decide how much we want - as distinct from free movement which we can't control. How do you get from that to saying we won't have foreign footballers? Dramatic statements are all well and good but are more persuasive if there is at least some basis in fact.