Merson, a pundit on Sky Sports' Soccer Saturday programme, was discussing the departure of Reds manager Roy Hodgson when he called Johnson's performances into question.
The England right-back, who trained with the rest of the squad at Melwood ahead of Sunday`s FA Cup third-round tie which will see the return of Kenny Dalglish as temporary boss, was obviously watching the programme.
And he chose to respond via his Twitter page.
"Comments from alcoholic drug abusers are not really gonna upset me and who is Paul Merson to judge players, he was average at the best of times," Johnson wrote. "The only reason he's on that show is coz he gambled all his money away. The clown!"
A number of people complained that Johnson had brought up Merson's former drug problems and the comments were soon deleted.