I never said that Valanciunas would be dominant. However, even without seeing him play in the NBA, his game is naturally a lot more aggressive when it comes to help-defense and shot blocking than Gray/Amir (ie: last year's starting C). He will need a season or two to fully adjust to the NBA game and will very likely be in foul trouble early and often, but his style of game is quite apparent. He won't dominate, but I still think that just his presence (ie: size, style of play, aggressiveness and hustle) will all be an improvement defensively over last season's starting C.
I think we can go by position, regardless of names:
PG - big upgrade defensively + 2
SG - same player +/- 0
SF - new player, similar style of play +/- 0 (though I personally think Fields will prove to be a +1)
PF - partly same player / partly no big difference +/- 0
C - small upgrade defensively + 1 (consistency and avoiding foul trouble will be a concern)