Wife is a student immigration adviser and she says Senegalese nationals (amongst many other nationals) have to undergo a TB test which can take 10+ days before the results are approved. Introduced by Theresa May apparently. No test, no visa. Apparently waiting times are worse in Sept due to the huge influxes of overseas students at these times. Don't know if being a highly paid footballer on a tier 2 sports person visa at a super wealthy football club can mean the process could be sped up at all.
This is, of course, complete guesswork on Sadio's situation but is a feasible reason for delay according to the missus. Bloody students.