Since the first team has four backup midfielders, it is not too surprising that Valverde is only starting three of them today. The choice was not Denis or Alena, it was Alena or Samper. I'm not sure why Alena would be preferred over Samper, given their relative amounts of La Liga experience.
Logical line up, would have liked Alena over Dembele but other than that can't complain. EV said already that he won't play more than 3 Barca B player in case injury happened and no way it wasn't gonna be all in defense
Normal that players who don't feature like Denis get those minutes against 3rd division team.