He did not need to say that and that argument is retarded.He didn't say ''Oh drats! now that AND ONLY NOW THAT I only have one hand Sasori is stronger! If only it had been different! And my clay! Oh if only I had my clay I could say Sasori was weaker than meee!''
That's not what he said though. He didn't say Sasori being stronger than him was a conditional statement nor did he imply it.
In context Kakashi decided to take on Deidara himself because he was the one who took down Kazekage and Deidara laughed it off with "sasori is stronger comment"