Balde is a good left back, and if not for the injury, he has the ability to become one of the best left backs in the world. He is good in attacking and defending, and he was one of the players who contributed to Barcelona's success last season. Balde was injured in training and will be out for four weeks, which is another problem for Barcelona because no other player will be able to do what he is doing. Gerard Martin can be managed, but he is not good enough to hold the position. If not Inigo Martinez last season, Barcelona would have lost many games when Balde was injured.
I am not a Barcelona fan, but I don't like it when players get injured because they are also human beings. Injuries have made some players look bad, which is why the Spanish coach isn't calling Balde during international competitions. Barcelona needs to get another left back to help Balde, if it's Gerard Martin, I don't think he can handle the pressure.
You are right that his skills in both attack and defense make him hard to replace and that Barcelona has big problem because they do not have good backup. Team reliance on one key player makes them vulnerable and it is huge risk. It is also true that injuries can hurt player reputation especially with national team coaches. I agree that it is tough to see any player get hurt and it will be interesting to see if Barcelona signs another quality player to solve this problem.