The former Manchester United man has entered into a war of words with the Liverpool defender over his decision to back Luis Suarez despite his alleged racial abuse of Patrice Evra
McGrath had previously criticised the England international for his decision to wear a t-shirt backing his Uruguayan team-mate during the aftermath of a spat with Patrice Evra.
However, the Liverpool full-back hit out at the ex-Republic of Ireland man this week, insisting that he had singled him out because he was black.
Johnson told The Daily Mail: "The McGrath thing, that’s actually racist. Saying what he said is racist.
"He is only saying that to me because I was the only black lad wearing the t-shirt. He’s targeting me because of my colour."
And now McGrath has warned the former Portsmouth man against reigniting the row which saw Liverpool widely condemned by observers.
"It’s sad someone is having a pop at me," McGrath said.
"I thought supporting a player - Evra - with the same colour skin might not be such a bad idea for Johnson. When he is 52 and not as popular as he is now, he might think differently.
"Liverpool got hammered in round one and, if Glen is starting round two, I think it’s a mistake.
"With Liverpool just winning a trophy, I’d imagine Kenny Dalglish would prefer to put all this to bed."