The allegation was made public on Radio Five's 606 phone-in show by caller Richard Bridge, who, amongst others, claimed that the Senegalese international spat in the direction of the away fans on more than one occasion.
Although the allegation was at first categorically denied by Liverpool on Saturday night, John stresses that he has liased with his counterparts at Anfield and says: "It is a matter for Liverpool Football Club to investigate and, after speaking to them, I am assured that they are taking the matter very seriously.
"I am satisfied that a full investigation is being carried out by the club and Merseyside police, and that several fans have been interviewed.
"If there are any other witnesses they can contact Liverpool or we will pass the details on ourselves."
Other Liverpool substitutes have not been implicated in the allegations, and in fact Steve Gerrard allowed himself, according to reports, an appreciative - if, perhaps, wry - smile when Hammers' supporters sang 'we're going to win 3-2' when faced with a two goal deficit.