The Spain international, who is set to leave Camp Nou this summer, appeared to refuse to take on the responsibility following Xavi's substitution at San Mames
Xavi skippered the Blaugrana at San Mames, but when he was withdrawn late in the game, the captain's armband was given to Valdes.
The Spain shot-stopper seemed to refuse to put it on, but he has now explained that he merely passed it on to team-mate Lionel Messi because it did not fit him.
"Ceding the captain's armband to Leo during yesterday's game was for no other reason than because of a problem with size of it," Valdes wrote on Facebook.
"By not being able to adjust comfortably to my arm I decided to transfer it in the middle of the game."
Valdes, who is a produce of Barca's youth academy, is set to leave Camp Nou this summer after rejecting a new contract with the Catalan giants.