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There is no bigger matchup than the one in the middle of the field between United's Tom Cleverley and Michael Carrick and Real Madrid's Sami Khedira and Xabi Alonso, as both teams vie for control in the center of the pitch.
Both Madrid and United could field different players in the middle for their respective home and away matches, but the four mentioned above are the most likely to take the field, meaning we can already tell how important this matchup is going to be.
Cleverley, in particular, has come a long way this season, and this Champions League tie will no doubt be his toughest fixture in the middle all season. The England international has shown he can be world-class at times—now he needs to succeed when United need him most.
For Madrid, there is no doubting the class of Khedira and Alonso, but Luka Modric, if Mourinho opts for the Croatian international, is highly competent as well. Modric has played against both players recently when he was at Tottenham Hotspur and could be a sneaky selection for Madrid if his form is good heading into the tie.
Either way, the battle for control in midfield will be key in determining which team's attack can get off the ground, which will, in turn, decide the winner of the match.