Carlo Ancelotti has turned down the chance to return to AC Milan, telling the club he needs rest.
Jose Mourinho insists Diego Costa is content at Chelsea in response to reports linking him with a return to Atletico Madrid.
The 34-year-old has agreed terms with Blues and passed a medical but the club are waiting on a visa application before a one-month deal is completed.
Former Senegal international Diop, who helped West Ham to promotion to the Barclays Premier League last season, has impressed on trial at St Andrew's.
He has also played in England for Fulham and Portsmouth, with whom he won the FA Cup in 2008.