The Argentina star has not featured this season due to a calf injury sustained in pre-season but was named in the travelling squad for Barca's opening UEFA Champions League group game at Signal Iduna Park.
Valverde has been pleased with Messi's training performances but will not make a decision on whether his captain will start the match until closer to kick-off.
"We'll decide before the game," he said. "It wasn't very clear a week ago if he could come, but he was with the team in the last few training sessions, finishing well, taking the fear of pain out of his mind. We'll see what we do."
Whether or not Messi returns, Barca could well hand a UEFA Champions League debut to Ansu Fati, the 16-year-old who has taken LaLiga by storm this season.
Fati became Barca's youngest ever league goalscorer against Osasuna last month and marked his full Camp Nou debut on Sunday with a goal and an assist in the first seven minutes of the 5-2 thrashing of Valencia.
"I've not spoken with him, save for tactical matters," Valverde admitted. "I don't like the history of giving advice. Of course I will talk with him, but you have to give him some breathing space.
"What he is doing is exceptional. If he plays, it's not so he can break a record, but because of the needs of the team. We love him."
Barca has only lost one of its past 28 group-stage matches but is 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."
Wednesday's game will see Barca come up against Paco Alcacer, who has flourished since leaving Barca initially on loan in 2018.
"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."