The Swiss trainer praised the recently-departed Germany international while also attempting to soften the expectations surrounding their new signing from Twente
The 54-year-old recently signed a two-year extension to his contract at the Bundesliga side that will see him remain at the club until the summer of 2015.
In an interview with Sueddeutsche Zeitung, Favre spoke openly of his admiration for Marco Reus following the player's move to Dortmund.
"[Reus] is totally impossible to replace. Totally impossible! He was involved in 90 per cent of our goals last season.
"He has so much potential that few people in Germany actually realise. His game is incredibly intelligent, he understands everything."
Favre also took the opportunity to attempt to lessen the hype surrounding Luuk de Jong's arrival at the club, following the Netherlands international's move from Twente.
"De Jong scored his goals in Holland. In Germany it is more difficult, the quality of the opponent is better here."
Speaking after a training session with his new club in Rottach-Egern, the striker addressed the pressure he feels having made the switch from the Eredivisie to the Bundesliga.
"The pressure is already there, to be honest, but I was top scorer for Twente in the Netherlands. I am happy that Borussia Monchengladbach spent so much money on me as I am worth it.
"I just want to be part of the team, to improve my ability and show my qualities. It does not matter if I score or assist goals, I am a team player, the team's success is paramount."
On the difference between football in the Netherlands and Germany that Favre mentioned, De Jong said: "We train here a bit more than in Holland, but that's not so bad for me because I must be a little tougher."