The Senegalese international, who appeared to complain the ball was not thrown back to him quickly enough, was questioned after the match at Everton's Goodison Park stadium.
"Merseyside police is investigating an allegation of a 28-year-old man using racist language at the Everton match on Sunday 20th September at Goodison Park," said a spokeswoman for Merseyside Police. "Enquiries are underway."
Diouf suffered abuse from Everton fans following the clash with the ball boy, and again when he went to take two corners in the area of the original altercation.
(Reporting by Kylie MacLellan, Editing by N.Ananthanarayanan)