I feel like we heard this before.
"Semedo is doing good and convinced Valverde that he is going to be a starter!" only for Bob to come back and play the next 5-6 games straight despite Semedo's performances.
I have little interest in defending Valverde but this requires a reply in depth or at least the mention of a few "facts".
The reality is that neither Roberto (less so) nor Semedo (for now that is) are world beaters and that the right back position has been "vacant" ever since Dani Alves (one of the best right backs in history and probably the best right back in FCB's history) left. Semedo is a fairly recent arrival whose greatest strength (compared to Roberto) is his defensive output and pace. Roberto on the other hand is (so far) a better offensive right back. So Valverde alternating between both players depending on the opponent is logical and not surprising. One can always discuss the timing though which I have often disagreed with as well. However as long as the position is not set in stone there will not be consistency.
Let us not forget that Roberto is a senior and a local Catalan canterano whose main position has been the right back for quite a few seasons now. His playing time is restricted to the right back position even more so considering the great depth of our midfield.
Having said that I don't rate Roberto as a right back much. In particularly defensively. However Semedo is not the finished article (of course for that to occur he will need continuous playing time and that is where the dillema emerges) but he has not exactly taken the world with storm and I otherwise like him as a player and I believe that he has the potential to reach a much higher level in particularly offensively.
Anyway another problem with our two right backs is also the fact that unless Dembélé is playing we do not have a genuine right winger/attacker when using the 4-3-3 formation as Messi drifts inside which also impacts Rakitic's contribution in the midfield.
That is why both Semedo and Roberto have performed better in a more compact 4-4-2 as I recall it last year. Not that this says a lot but nevertheless.