Alcacer has not been utilised by Valverde since he played just over half an hour as a substitute in a 2-0 win at Deportivo Alaves on August 26.
"Paco was in the squad for the Girona game. Who knows what will happen in the future?" Valverde told reporters ahead of the trip to Estadio Jose Alvalade on Wednesday.
"There are 24 players in the squad and you'll ask me every week about the players who aren't in the team.
"I can't pick them all. UEFA won't allow that."
By contrast, Andres Iniesta has started all of Barcelona's last six games, and Valverde underlined his importance to the side when asked whether the veteran midfielder would be used more sparingly this season.
Iniesta's 23 La Liga appearances in 2016-17 was his lowest total in a single campaign since 2003-04, but Valverde appears to be happier to rely on the 33-year-old than his predecessor, Luis Enrique.
"Andres is clearly a vital player for us, which is why he's playing so many games," said Valverde. "It would be a lie to say we don't miss him when he's not on the pitch.
"He's one of our playmakers and he will be an important part of our team. There's no doubt he'll be playing a lot of football this year. We just hope he can continue to play at this level."