Dortmund host La Liga champions Barca in their Group F opener, with Messi in contention to make his first appearance of the season.
The Argentine superstar has missed Barca's opening four La Liga fixtures with a calf problem but has been included in Ernesto Valverde's squad despite the coach suggesting on Friday the 32-year-old would not be ready in time.
It remains to be seen whether Messi will be able to start, though Dortmund captain Reus is hopeful the diminutive talisman does make an appearance as the Bundesliga side aim to test themselves against the best Europe has to offer.
"We want Messi to play, but we cannot influence what tomorrow will look like," Germany international Reus told a media conference on Monday.
"We do not want to talk that much about the opponent. Everyone knows that he is the best, but we play at home and want to win.
"A difficult game awaits us. We have to be careful and will we have to suffer a lot in the game. We have to be brave and play our game, put ourselves forward and then hope to score goals."
Paco Alcacer could be key for Dortmund as the Spain striker goes up against his former club for the first time.
Alcacer spent two seasons at Camp Nou, struggling to secure a regular spot following a move from fellow La Liga club Valencia.
The 26-year-old enjoyed a fantastic maiden season at Dortmund, scoring 18 Bundesliga goals in 26 outings, and has netted five times in four league appearances this term.
Reus is backing the striker – who accused "many people" at Barca of treating him badly during his stint in Catalonia – to impress against his former employers.
"I've talked to him a few times. He is calm," said Reus. "Of course, he is looking forward to it because he has spent nice years there, even if he had not too much game time.
"I hope he can help us in the game. He is an important player. He plays smart on the pitch, he knows where the next man is.
"He is an ice-cold top scorer who does not need many chances, his statistics are outstanding. But even for him there is room for improvement, as with any player."