Valverde was speaking after Athletic started their UEFA Champions League Group H campaign with a 0-0 draw against Ukrainian champions Shakhtar Donetsk at San Mames on Wednesday.
Mikel Rico and Ander Iturraspe were employed in central midfield for the fixture and the pair enjoyed plenty of the ball, but Athletic's cutting edge in the attacking third of the pitch was too often left wanting.
And Valverde concedes that his midfield remains a work in progress following Herrera's close-season departure to Premier League giants Manchester United.
"I have different possibilities for the position occupied by Herrera," he said. "I'm doing tests."
Reflecting on Wednesday's match, Valverde admits his men were frustrated by a Shakhtar side who refused to let Athletic play their natural game and he was ultimately happy to settle for a point.
"I see the draw from a positive point of view," he added. "We have faced a great team.
"We have suffered for the point. We could not play our game. We have committed more errors than we would have desired."