What Alba has always excelled at is looking for the playmakers and making disguised well-timed runs from deep (which Messi would always find), using his pace to break behind the back line. It's what your creative players need to actually be creative— without options, playmakers just have to recirculate the ball. On the other hand, what Alba does with the ball after he gets it is always hit or miss— more often miss. He tries to put in a square ball or a cutback and, more often than not, blasts the ball into his marker. I came to expect Alba's final balls to be mostly as useless as Alves' multiple targetless crosses. But the times Alba connects (like today), it can change the game. He was excellent today.
His other great skill is volleys. He has great technique and usually forces a save. He really should shoot more, but years of trying to feed Messi has trained it out of him.