The left back area is probably the weakest part of Barca's team so no doubt SAF will look to attack that and I think Nani will get the nod because he provides a better cutting edge going forward on that right side.
...Park and Valencia are just more hard-working and better at tracking back than him
People forget - probably because of the return from injury of Valencia and the rotation of him and Nani, just how fantastic he was earlier in the season, single-handedly winning us games and averaging an assist or goal per game for the first half of the season.
I actually don't think Nani will start against Barcelona. Park and Valencia are better options as they are more discipline and provide better solidity on the flanks. On the other hand, Nani sometimes tend to lose the ball a little bit too easily when he tries to do too much with the ball and that is not something you want against a team like Barcelona.
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I doubt that if Abidal starts at left back. On the opposite side, Alves is a little bit of a suspect to the odd error IMO.
Anyway, I just read this article and there are one or two things to probably take note.
Why picking on Piqu? could be the key to beating Barcelona | Paul Hayward | Guardian
I wouldn't start Nani - he'd make a great impact sub later on. What about up front though - Rooney/Hernandez or Rooney/Berbatov?