He played in the Roma fiasco if I recall correctly, because Valverde used long balls on him to win headers vs Kolarov. I remember specifically laughing very hard at that 'tactic'. Used him as a wide midfielder in a 4-4-2. And Roberto had a terrible game vs Madrid in the 2-2 last year.
Well, I have pretty much tried to erase that horrendous game from my memory (although that Kolarov goal is the only thing that I recall from that game) and I don't really bother to look whether he played or not.
The point being is that Roberto tends to perform great (overall) in clásicos and that he usually performs well in top games too. He is not a world beater but neither is Semedo really. I expected much more from Semedo here almost 2 seasons into his Barça career. He is approaching 26 years so not exactly a youngster either. I believe that Semedo has room for improvement but we could do better on that RB position. Finding another Alves won't happen anytime soon but less could do.
When Roberto has a great game he is almost as dangerous offensively as Alba is. That gives us two flanks to create from rather than the one (left one) we tend to have whenever Dembélé is not playing as a RW. This element cannot be underestimated. Yet to see Semedo making the entire "right flank" his. I want to see a more daring Semedo offensively as he has the tools for his. In particular pace. I don't know if it is a mental thing or if he is just not good enough technically or in terms of combining with other players. Roberto is clearly better than him in this rather crucial department. On the other hand Roberto is a makeshift RB and lacks (often) the positioning (although it is getting better naturally) and the brute pace (although he is not slow by any means) and strength of Semedo.
So we are back to square one. Play them depending on their match fitness level, their form, who they will play with next to (Piqué, Rakitic or Dembélé upfront) and what the strengths and weaknesses of our opponent are.
@ Semedo fanboys needs to be put on suicide watch. They can't accept that Semedo got destroyed by a teenager while Roberto had better performance against the teenager. Excuses Excuses 
