The Scottish champions is looking to defend its 2-1 lead from the first game at Parkhead last week.
Brown's hip injury kept him out of the 1-0 Scottish Premier League win over Aberdeen last weekend.
Forrest also missed the league's curtain raiser and is battling to recover from a groin injury.
At the press conference in
"They are still a little bit stiff and sore.
"We are giving them as much time as we can, and maybe even give them until tomorrow afternoon.
"I think there has been a significant improvement in both of them over the last couple of days.
"Having spoken to the medical team, yes (Brown's chances are good). Scott likes to play even when he is not 100 per cent.
"He is such an important player for us but because it is such an important game, we can't take too many chances.
"And it is still so early in the season as well. We don't want to lose him if we push him too hard, too early."
UEFA Champions League
Borussia Dortmund centre-back