Actually Hull would have still gone down if Silva had of averaged his results out over a season, they’d have finished level with Watford on 40 points but had a worse goal difference.
Looking at it they were 2 points clear of Swansea with 3 games to play and lost all 3 against Sunderland (h) 0-2, Palace (a) 4-0 and Spurs (h) 1-7. Food for thought about the concerns over Silvas ability to organise a defence I guess given how open we’ve been ourselves all season.
It's difficult to put those last three results in context. The Sunderland game was against an already relegated team (who of course always suddenly remember how to play once out), albeit one they needed to win to stay up. The Palace game was away to a resurgent Allardyce stay up specialist team, which they went into knowing even if they won, and beat the outstanding Spurs in the final game, they might still go down on GD. As soon as Palace scored, you can see why their belief was destroyed, and the Spurs loss is easy to understand.
That said your point about goal difference is still fair, since even without those last two big losses they would still have been on worse GD in all likelihood. Whether not conceding with Hull is a fair measure is another question, particular having lost influential midfielders mid season, so maybe his Watford results are a better measure. They aren't that bad, other than a few games against free scoring big teams, and of course our freak comeback (without their good keeper for the key bit). There will more than a couple of teams that concede lots to City and Chelsea as well as Liverpool, particularly away from home. I'm not sure it's possible to draw any conclusions about whether he could get our team defending better as yet.
If we don't get him shortly, the season at Watford with average level PL defenders show give a fair indication.