Despite winning their last two fixtures in La Liga, Athletic have been desperately poor this season, with the 1-0 triumph over Almeria on October 25 ending a run of nine matches without a win in all competitions.
Riding the momentum of Sunday's 1-0 victory at home to high-flying Sevilla, Athletic had been looking to beat Porto in order to keep their slim hopes of UEFA Champions League progression alive.
But goals from Jackson Martinez and Yacine Brahimi earned Julen Lopetegui's side a deserved win and Valverde acknowledged that his team had no chance as soon as they fell behind in the 56th minute.
"It is hard [to take the defeat]. They were better than us," Valverde told reporters. "We don't have a chance of going through now. We will fight to finish third.
"They [Porto] were very strong and are a great side. They won their battles, we didn't win the second balls.
"In the second half we came out better, but once the first goal went in it was over. Now we have to fight for third place."
Athletic's final Group H games see them host third-place rivals BATE after heading to Shakhtar Donetsk, with a place in the knockout phase now out of their reach.
