We all know Rafa likes to make changes, often using completely different tactics from week to week. I think all four are needed to accommodate these changes. Gerrard and Alonso is a no go for me, they're great players individually but together they don't seem to have the right balance. In one aspect their style is similar, their passing. They both tend to spray 30-40 yard passes about, and when they're both on the pitch our distribution becomes a bit predictable. The best partner for Gerrard would be Mascherano for me. Mascherano's a great holding player, winning the ball & playing it simple most of the time, his style would work well to keep the balance whilst Gerrard would play what I feel is his best role, a box to box midfielder. He is too good at everything to be restricted to being the 'holding' player or playing in the 'hole', as both roles take something away from his game, be it offensively or defensively. The approach could be a bit too gung-ho for Benitez when we're playing top opposition and he might go for a more defensive pairing/trio in those matches.
Sissoko, as you rightly say, will probably not play as many games as the other three, but I'd still expect him to get about 30 games including sub appearances. He is very useful against good passing sides as he doesn't give them time on the ball. He chases down relentlessly, and often they resort to route one football to try & bypass the midfield, which becomes a lot easier to defend against. The thing that lets Sissoko down though is his passing, he can't seem to hit an accurate pass more than 5-10 yards. This is why I feel Alonso is the right partner for him, as he reads Sissoko's game better than either of the other two. When the two of them play together you notice Alonso almost trailing Sissoko when the opposition have possession, ready for him to win it & supplying a simple option for him to pass to, relieving Sissoko of the duty of too much serious passing. Sissoko can also be used as a good sub late on in the game as his exceptional energy levels can exploit tiredness in the opposition.
Alonso and Mascherano is a very defensive pairing, and I feel the time to judge this option is when we've got some better wingers in. With both playing holding roles you need very attacking wide men, and at least one target man upfront. As I haven't seen it played with the right wide men in place I can't really comment on it's effectiveness, but without the right wingers it hasn't worked for me. I feel Mascherano & Alonso are more effective when used as part of a trio with Gerrard, either in a 4-5-1 or a 3-5-2. I think 3-5-2 would need us to buy a new right wingback, and that's not really a priority right now, but 4-5-1 will probably be used a fair bit in big cup matches, especially away legs. The formation gives us essentially three defensive midfielders when needed making a back 7 to break down, but gives us a great attacking outlet through Gerrard to attempt to nick the treasured away goal.
Gerrard will still play the occasional game on the right I believe. I would hope that we sign two wingers, both very attacking. There will be occasions that you want someone more defensive on the flanks, Riise can do the job on the left & Gerrard can do the job on the right. I would hope with new options arriving on the wings that Gerrard would only play a handful of games there though.
Lucas Leiva's hard for me to comment on, as I've only actually watched him play one game in the Copa Libatores, but from what I've heard & seen in that one game he seems to be a Sissoko type player, harrying the opposition and winning the ball, but with a shot on him as well. If the lad can settle in well & do as well as my Brazilian friend has insisted he will do then I would see Sissoko going out of the door. Personally I would wait until January to see what the kid's like, and try not to play Sissoko in the Champions League until then to keep his value.
One thing's for certain, and that's that we don't need any strengthening in central midfield. I feel that all four of them will play a significant role this season though, and with the right players coming in for other positions we will (hopefully) have a great season.
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