I don't really see Blind as just a LB back up, considering he's only played 5 games as a left back, and 20 as a midfielder, and a large number of those games in midfield, he played very well (Check his threadmarks from before he got moved to LB, which was from the Spurs game at home. The caf was gushing over him as a DM). Many were calling him the best signing of the summer for most of the season, barring the last 3 or 4 games.
He had a few ropey games in midfield towards the end of the season, and now he's shit and should never play in midfield? Nevermind the 20-odd good performances. Those 3 or 4 iffy ones and damned him from his best position forever, it seems. Short memories all round.
He's not as good as Carrick (though plenty on here were saying the opposite earlier in the season) but that's not really a slight on Blind. Carrick is incredibly good at what he does, and it's plausible that Blind could reach that level.