Barca won 2-0 thanks to two goals from Lionel Messi, who also missed a penalty.
"Alaves were so compact that their forward was closer to our midfielders than defenders," Valverde explained in his post-match press conference.
"We've overcome a difficult test.
"It took a lot to open the scoring.
"There were stuck within a 15-metre space and it meant that sometimes we were so close that there were as many as 18 players in such a small space."