Police sources suggest that over the next 48 hours the Goodison Park ball-boy who has made the allegation will also be questioned.
Blackburn boss Sam Allardyce was left fuming by his side's abject performance in the 3-0 defeat, and now he has the added problem of this latest allegation against the controversial Senegal international.
Diouf was interview by police, accompanied by a Blackburn official, in a private room at Goodison Park an hour after Sunday's match.
The Senegalese international was seemingly involved in an altercation with the ball-boy during the first half, Diouf then suffered abuse from home fans, and again when he went to take two corners in the area of the original incident.
A police spokesman said: "Merseyside Police are looking into an allegation that a 28-year-old man used racist language at the Everton match."