Both are at risk of missing the trip to the Allianz Arena, with Favre's men looking to keep the pressure on leaders Borussia Monchengladbach and potentially go four points clear of Bayern, who will be coached by Hansi Flick for the first time in the Bundesliga.
Favre confirmed Reus and Sancho will be assessed on Friday and seemed reluctant to risk either.
"We have to wait for both," Favre told a news conference. "We do not know if they'll train or not, we'll see tomorrow [Friday].
"We have two or three players still in that situation. We need them to be fit and then we'll decide which team to play in which system. A player has to be fit, no matter what the game."
Dortmund have a wretched recent record against Bayern, having not won a league game at the Allianz Arena since April 2014 and lost their two most recent trips 5-0 and 6-0.
But Favre insists BVB are not scared of the club that sacked Niko Kovac on Sunday as a result of their 5-1 humiliation by Eintracht Frankfurt, with his team seemingly back on track with three straight victories after a run of two wins in eight games.
"We have always stayed positive and have looked forward," Favre said. "It was a difficult phase, and if we draw once away, that is the end of the world in Dortmund. It was not as bad as it was said.
"It has been very difficult here in the last five years, we have nothing to lose, we respect Bayern a lot, they are a top team, but we are not afraid."
Dortmund sporting director Michael Zorc added: "[Der Klassiker] is a bit special, it's very important clubs in the Bundesliga, it's a very important game.
"For many, it's the most interesting game of the season. It is a very special game. You can tell that in build-up and reaction, because the game has a special impact on the future."