Definitely a contributing factor but I don't think that's what it should be put completely down to. Does fitness or double marking actually effect his positioning. IMO when he began to struggle his positioning off the ball became too predictable and allowed himself to be pinned to the sideline. Its like he was zapped of imagination, deciding to run down the line and beat his markers for pace instead of trying to change direction and catch his marker by surprise. I think that mainly came down to confidence and maybe his fitness. You see him in that video able to take on 2, 3 or even 4 players but when you run down the sideline every single time you get the ball or a passing midfielder gets the ball you're depriving yourself of that unpredictability. Moreover, one of the only times I remember Krasic actually sitting wide and high up the pitch and finding himself on the end of a successful pass was his last second goal against Lazio. Otherwise he retrieved the ball himself in midfield and ran it up the pitch himself. THAT'S when he truly succeeded, not when he tried to sit high and wide and find himself on the end of a good pass like he would in a 4-2-4.
I was trying to explain that a while ago but I ended up confusing myself. Krasic won't be best when he plays high up the pitch in a 4-2-4 or 4-3-3 because he's too easy to mark that way. A 4-4-2 suits him when he's allowed to roam deep and free, find the ball himself and take on players with more space allowed to him.
I hope that makes some sense.