Mascherano is stronger than Blind, and faster, too. And teams are scared of pushing too high against Barca for fear of being overwhelmed at the back with that famed front line. And there is also the fact that the Premier League is more physical than the La Liga.
When teams will play big strong pacey attackers upfront or press our midfield and defence and force mistakes or start putting in crosses for multiple target men, Blind will be exposed. He doesn't belong in central defence in this league for a club like United.
We rely too much on our full backs to keep opposition wingers quiet and the long balls over defence minimal. Blind's shortcomings are being covered by the form of the other three in defence. I agree with the fact that if our full backs are on the offensive chasing a game and get caught, Blind's lack of pace and height will be a grave issue.
It can't be denied that he does add the extra composure and passing efficiency to the defence. 98.4%, as imperi pointed out. As long as there are other people covering up for his weaknesses, he is doing what he does best - pass the ball around intelligently, make well timed tackles and position himself well. But I'm afraid he's just a tactical trap away from a disaster. The top PL teams will expose him. I'd rather we'd signed a proper ball playing CB in his place.