The Argentina star is set to miss the next six weeks due to a hamstring injury sustained in the 4-1 win over Real Betis - his third lay-off of the season already.
Although Valdes admits his side are likely to miss the attacker's influence during his absence, he is confident the 26-year-old will recover swiftly from the problem.
The goalkeeper said: "He is the best there is and he always will be, for what he has given to this team and what he has made me experience as his team-mate.
"For me, Barca will always depend on him because he is the best in the world.
"To me he seems sad. By defect we players are selfish and we can't understand it when we can't play.
"Knowing him and the winner he is I have no doubt what he will now put into recovering, in fact he is already doing so."
The Argentine has scored 14 goals in 16 outings under Gerardo Martino this term, having fired Barca to Liga glory last season, scoring 46 times in 32 games.