Barca have only lost one of their past 28 group-stage matches but are under huge pressure to perform this season after collapsing in the semi-finals to throw away a 3-0 first-leg lead over Liverpool.
Valverde believes Dortmund will offer a good litmus test for his side, as he looks to put the disappointment of the 4-0 loss at Anfield and the dramatic quarter-final exit to Roma the year before behind him.
"I expect a rival of the highest level, a challenger for everything in their country and in Europe, and a good yardstick for us," he said.
"It's a competition in which one bad day can knock you out, even if you have had a great competition until that moment. The two teams in the final last year [Liverpool and Tottenham] lost more games than we did. You can't neglect even 10 minutes."
"Alcacer is a great player and he's showing that for Dortmund and the national team," Valverde said. "He's playing regularly with Dortmund. What we know about him is both an advantage and a disadvantage."