Hazard made the £89million move to Spain from Chelsea last summer in a bid to fill the gap left by the departure of the five-time Ballon d’Or winner.
Belgium captain Hazard, 28, has struggled with his fitness since joining Los Blancos and admitted arriving eleven pounds overweight to pre-season.
Messi believes it is unfair to directly compare the two players.
In an interview with Marca, he said: “It is very difficult to replace Cristiano, but Hazard is also a great player.
“Hazard has a lot of quality. He is a different football player who can unbalance [defences], but I think he is different to Cristiano, with different characteristics.”
Hazard scored 85 Premier League goals in 245 appearances for Chelsea but has only scored once in the league for Real in his first eight La Liga appearances of this season.
The Belgian has replaced Ronaldo as Real’s No 7 and has huge boots to fill on the pitch at the Bernabeu.Ronaldo scored 311 league goals in just 292 La Liga games and sits second on the all-time Spanish league goalscorers list, with only Messi ahead of him.
Barcelona superstar Messi won a record sixth Ballon d’Or in December, taking his tally one ahead of his former La Liga rival.
The two went head-to-head in the Spanish league for the duration of Ronaldo’s nine-year stint with the Galacticos and only once was La Liga’s top scorer not one of the two.
Messi topped the La Liga scoring charts five times to Ronaldo’s three, with the Argentinian’s Barca team-mate Luis Suarez taking the accolade in 2015-16.
Hazard suffered a fractured ankle against Paris Saint-Germain in the Champions League at the beginning of December and is unlikely to feature against Messi in tonight’s El Clasico.
Messi said that when Barcelona host Real at the Nou Camp the games tend to be tighter and more cagey affairs than the corresponding fixture at the Bernabeu.