Unpopular view here.
He was bad and predictable but not as bad as some here want to make it appear like, especially when the entire team was shit and those bestowed with the gamut of leadership were either too nervous to lead or being shafted in their own positions too.
Martin,Araujo,Lopez and tosome extent, Gavi, were the weak links in the team. How it took Kovac an entire half and 15 minutes to figure out that Martin was the weakest link in Barca's armour is still a question he must answer if I were part of the leadership at Dortmund. It was pretty obvious Martin was a hole in the defence right from the start and any good coach should have picked that up before the match if he reviewed Barca's last macth vs Leganes. I am pretty sure his answer would circle around the fact that they overestimated Barca until they found out they were ordinary pussies as the match wore on.
That said, the situation is still not all that hopeless especially re: Martin, considering he will have to play next few matches in place of Balde unless Flick finds an ingenious solution or Balde miraculously recovers ahead of time. It is obvious he lacks the technicals to play like Balde or anything close to it but he makes up for it with a stronger physicality and aerial presence. It is left for Flick to do a SWOT and adjust the team's arrangement on the left side to make use of his strengths more and cover his weaknesses. I am not going to bother my head about how Flick figures that out, he is the one earning the millions, not me.