Barcelona's team we know; Manchester United's is a matter of speculation, a fact that, in itself, is indicative of two things. First, that Barcelona is the favorite, with such a defined and familiar style of play that, even in this age of rotation, it is possible, as with the greats of the past, to rattle through a first eleven.
But secondly, it suggests how fortunate Barcelona has been with injury this season. Its training methods are clearly superb, and its players regularly run on average a mile or so more than opponents in the same game, and the club deserves great credit for that, but still, there is an element of fortune that nobody has caught their studs in the turf while turning, or taken a whack on the knee or ankle at the wrong angle.
Assuming nothing untoward happens in training this week, the only doubt is at left back. Eric Abidal, having had a tumor removed from his liver in March -- the one Barca player to suffer misfortune with his health, and even he has recovered staggeringly and happily quickly -- may be fit, and both Adriano and Maxwell returned to action after injury in the win over Malaga on Saturday. It could be that Carles Puyol plays at left back with Javier Mascherano in the center, but it seems more likely that one of the three left backs will be fit and Puyol can operate alongside Gerard Pique in the middle of defense.
So Barcelona will probably line up in a familiar 4-3-3: Victor Valdes in goal, Dani Alves at right back, Puyol and Pique in the center, and one of Abidal, Maxwell and Adriano at left back. Sergio Busquets will hold just in front of the back four, the player who initiates most attacks, with Xavi and Andres Iniesta just in front of him. Pedro will play on the right, David Villa on the left, with Lionel Messi as the false nine, the center forward who drops deep to link with the midfield.
With United, there are rather more debates, so let's for a moment forget personnel and apply first principles. Do that, forget practicalities, and something rather odd emerges. The basic rule is that you need as many players at the back as the opponent has forwards, plus one. Roughly speaking, Barca plays a 4-3-3, so that would suggest a back four. Except that Lionel Messi, the central player of the three, is a false nine, dropping off toward midfield, leaving space into which David Villa and Pedro can break from wide. That is a conundrum with which defenses have struggled all season; nature may abhor a vacuum, but center backs abhor them even more. What should they do? Follow the forward and risk opening gaps, or leave him alone to -- hopefully -- be picked up by the holding midfielders, who themselves have Xavi and Iniesta to look after?
Well, maybe the answer is to do neither. If Barca plays without an orthodox central striker, perhaps the answer against them is to play without an orthodox center back -- or rather, to play with only one, the "plus one," and to track Messi with either a designated man-marker or an additional holding midfielder. That would then leave a back three, but unlike the familiar back three, it would consist not of three central defenders, but of a center back and two fullbacks (who would, admittedly, have to play slightly tucked in to match the angles on which Villa and Pedro attack).
There would then be three holding midfielders (or perhaps two holders plus a man-marker) to handle Messi, Xavi and Iniesta). So, three defenders, three holding players -- that leaves four. Against Barcelona, with its attacking fullbacks, wide men are essential, and given Sergio Busquets is the metronome who sets Barca's rhythm, the ideal would be an attacking midfield-cum-second striker to sit on him. That leaves one remaining player, ideally a rapid center forward to disrupt Barca's back four with his movement and look to use his pace to get behind them, perhaps making them wary about playing a full press.
So, if you had a limitless pool of players from which to draw, a 3-3-3-1, with the middle three narrower than the other two threes, would seem the ideal. That, though, is such a radical suggestion as to be unworkable. No manager is ever going to introduce a new formation without weeks of preparation. Besides which, the central defensive pairing of Rio Ferdinand and Nemanja Vidic is so good that there is no sense in disrupting it. In other regards, though, United can draw comfort from the way their players fit the paradigm.
Sir Alex Ferguson's most intriguing decision comes at the front. Does he stick with the Javier Hernandez-Wayne Rooney partnership that has proved so effectively recently that the Premier League's top-scorer, Dimitar Berbatov, has been relegated to the bench, or does he opt to play an extra midfielder and use Rooney as a lone striker? Either is possible, but Hernandez's pace on the break -- and given Barca has averaged over 70 percent possession this season -- and Rooney's form in the withdrawn position would seem to swing the argument in favor of the pair. Playing split forwards also means Rooney can sit on Busquets -- a vital role given the way Busquets sets Barca's tempo -- note how much more fluent Barca are with him rather than Mascherano in the holding role.
Park Ji-sung will surely be used on the left to try to check the runs of Dani Alves. It's commonly said that the space behind Alves is a potential weakness for Barca, yet nobody has really yet been able to exploit it; the best option may simply be to try to track his runs, and Park, as almost a specialist defensive forward, is probably as equipped as anybody to do so. Nani began the season well, but after his return from a broken leg, Antonio Valencia is likely to get the nod on the right. He will have a key attacking role in trying to exploit a fullback who may not be fully fit, but will also have to get back to provide additional cover for whoever plays at right back. Fabio and Rafael offer pace and attacking thrust; John O'Shea is the more solid option.
And so to Ferguson's other key decision: who plays in midfield? Ryan Giggs and Michael Carrick played in both legs of the quarterfinal against Chelsea and in the first leg of the semifinal against Schalke 04, but the worry is that they don't offer sufficient bite. If Darren Fletcher is fully fit -- and he managed the whole game against Blackpool on Sunday -- he could return, presumably ahead of Giggs. The feeling may be, though, that positional awareness is more important than aggression against Barcelona, and that Rooney, dropping deep from his trequartista position, provides the necessary snap. If Ferguson goes with a 4-3-3, though, without Hernandez, then Fletcher -- or Fabio or Rafael if he is not fit -- can come in alongside Giggs and Carrick.
The likelihood, though, is for United to play their 4-4-1-1, to look to contain Barca as best it can, and then to try to steal something on the break or from a set-piece. It's not United's usual way, but against Barca it's probably the only way.
Jonathan Wilson is the author of Inverting the Pyramid; Behind the Curtain; Sunderland: A Club Transformed; and The Anatomy of England.