The 27-year-old is now regarded as one of the best players in Europe and has been persistently linked with a move to Real Madrid.
Hazard said earlier this year that it was his dream to play for Real and recently claimed he would make a decision at the end of the season on whether he would leave Chelsea.
But Arsenal hero Merson believes Hazard has already made his mind up and is set to leave Stamford Bridge next summer.
Asked if he thinks Hazard has already made a final decision over his future, Merson told Sky Sports: I would say yes.
I would say yes because when youre talking like that its just leaving the door open.
I think he will play and enjoy his football like he is at the moment. He is the best player in the Premier League in my opinion.
I think hes the only player in the league who can get the ball on the halfway line and beat three players and score on a consistent basis.
Hes up there in the top five in the world of football.
Real Madrid, I think when they come knocking, or even Barcelona because I dont think [Ousmane] Dembele has really done it, I think he will be on his way.
As a Chelsea fan I just dont think you could knock him if he went.
Hazard, 27, is enjoying one of his finest seasons for Chelsea and scored a brace on Boxing Day to help the Blues beat Watford.