Alba- I am a LB and I score goals
Jordi Alba-The left back of FC Barcelona  has a devastating pace. Here is the video that shows his speed  and most importantly it shows, how apt he is for Barcelona.
Jordi alba Pace

     Another player who can make a difference while playing against teams which defend very deep. Sometimes its an advantage and sometimes its a disadvantage. Considering how much he contributes for a team in terms of attacking , we cannot expect the same from him in terms of defending. If any team follows a particular way of playing, the opponents will eventually figure out the weak link in the team. And that weak link, which is targeted by most of the teams is Jordi Alba.
       Jordi Alba covers whole left half of a pitch during a match. when Barca loses the ball, he runs back 80-90m behind to defend. The weak link/links of  Barca  are exposed when Barca face teams , who defend as a team, and try to minimize the space for xavi, Iniesta and Busquets. Those teams rely on Good  counter attacking too. If  both  Full backs  are in  the attack and if Barca loses the ball , there will surely be a big problem as we saw against Real Madrid in the Copa Del Rey (2012-13).
  The center backs look haphazard, not knowing when to tackle.Eventually, Poor Valdes gets all the Blame.
Luis felipe Scolari assigned Hulk to exploit the  space  left by Alba  and make things hard for him  (Hulk is physically strong than Alba OBVIOUSLY). Hulk easily won the aerial duels with Alba. You can see the video as how Jordi Alba was targeted by the Brazilians. There is a particular instance where Luiz gave a long pass to Hulk(As there is a high chance that Hulk will receive  the long pass because Alba is marking him)
See here :

 Brazil vs spain Tactical Analysis by kishorefcb  

The same Happened in the league game vs Valencia  :
See Valencia attacked 44% through the Right side.i.e Jordi Alba's  side
Iniesta, Neymar and Alba will be on this side of the pitch and Neymar doesn't track back well.
Credits:whoscored.com

 This little thing will matter the most, especially against Real Madrid. Valencia for sure, tried to exploit this,  but thanks to Messi's three goals at Mestalla, which guaranteed all the 3 points. If our attacking wins us the matches in the La Liga, there will be  a problem in CL for sure. AC Milan and Celtic  defend very deep and will surely look to beat us through the weak link/links. Here is the Highlights of Jordi Alba vs Valencia (First half)