Northumbria police have begun investigations after James McClean received death threats on Twitter after opting not to wear a Remembrance Day Poppy during Sunderland’s 2-1 defeat away to Everton on November 10.
The 23-year-old was booed by a section of his own fans when he came off the substitutes’ bench during the 3-1 victory at Fulham on Sunday.
A police spokesperson told Sky Sports: "Police are carrying out inquiries following a third-party report of threats made on social-networking sites. Officers have spoken to Sunderland AFC and inquiries are ongoing."
Black Cats boss Martin O'Neill commented on the issue after Sunday's game, and staunchly backed his player.
"James will deal with it. It's to do with the issue of last week [not wearing a poppy]. It's a free choice here in this world," O'Neill told reporters.
"James has lived with a lot of things, he's getting death threats now."
Sunderland claimed a much-needed victory against the Cottagers, with the hosts reduced to 10 men in the first half following Brede Hangeland dismissal for a challenge on Lee Cattermole.
Steven Fletcher, Carlos Cuellar and Stephane Sessegnon all found the back of the net for the away side to give the scoreline a convincing look, but O'Neill admitted that the sending off played a major part in his team's victory.
He added: "The sending off of course changes the dynamic of matches. You saw it yesterday with Tottenham and you have to try and take advantage of that and I thought we were able to do that in the second half yet we were never able to put Fulham away.
"We've been given the directives. I didn't know how many times we need to be told about it. We've been guilty ourselves with Cattermole getting sent off but sometimes I don't understand players now.
"There is a better chance of winning a football match with all your players."