The two sides meet in the semi-final of the Champions League this week and the Spanish goalkeeper has praised his Nou Camp teammate's ability.
However, Valdes has said that Roberto Di Matteo's men will have to contend with more than just the Argentine ace.
"Messi is number one in the world — that's unquestionable," said Valdes. "Every rival fears him but if you develop a system to stop him, that makes it better for his fellow strikers and that's a big advantage for us.
"To stop Leo is always very difficult because he is incredible in every game and scores so many goals."
Messi has notched 63 goals in all competitions so far this season.