Ravet was sent off just before the half-hour mark at Schwarzwald-Stadion for a lunging tackle on Schmelzer, referee Benjamin Cortus producing a red card, having initially booked the Freiburg debutant, before the Video Assistant Referee intervened.
Despite their numerical advantage, Peter Bosz's men could not find a way through and laboured to a 0-0 draw.
Schmelzer was making his first appearance of the season after overcoming another ankle injury, and is set to miss Champions League clashes with Tottenham and Real Madrid, as well as domestic fixtures including a home meeting with RB Leipzig.
Gonzalo Castro was critical of Cortus for setting the tone by failing to strictly punish earlier challenges.
"This is the problem: if you, as a referee, do not give an early sign with a yellow card then something happens, like this foul, and one of us is hurt again," he said.
Marc Bartra also had to be withdrawn early in the Freiburg draw due to a thigh strain, but he could make the trip to Wembley on Wednesday to face Spurs.