This is what happens in this forum. People tend to build pedestals for players after one good performance, then proceed to take sledgehammers to said pedestals after one bad one. When you read comments like Alba is the white Abidal, the only thing you can do is
Jordi Alba is an excellent player, and will be a staple of the team for years and years. However, defense is definitely not his strongest aspect. He has plenty of speed to track back and intercept balls, but he already has a handicap on physicality as he is very slight and smallish, then you add his overall defensive skills are still developing and suffer from some deficiencies. Offside traps are a particular weakness of his. What he is, is an extremely dynamic player that gives you much more on offense, than he takes away on defense because even with his deficiencies, he is still a perfectly capable defender that will improve with time.
Alaba is a stronger defensive LB than Alba, even though he is just 21 years old. He has more physical tools to stand up to the strongest players, whereas you've seen Alba struggling having to cover someone like Diego Costa.
Alba however, is much more polished and technical on offense.
Both are great young players, and IMO both are exactly the best fit for their respective teams.