Mascherano seems the logical one
Sergio Busquet's injury against Villarreal means he will miss the next few matches. Now Luis Enrique has the job of looking for a replacement against Rayo and Eibar... and maybe also against Manchester City and Real Madrid
These are Luis Enrique's options:
Inside the first team squad
Mascherano could become the central defensive midfielder, as he has done many times before. He did it 8 times this season and would be the most obvious choice.
Four times in the league (Eibar, Valencia, Elche and Granada) twice in Europe (Ajax and Apoel) and twice in the Cup (Elche and Villarreal). Barça has always won when Mascherano has played in this position.
Rakitic has already played in that position with Croatia, playing along Luka Modric. Barça's central defensive midfielder is different in this club than any other club in the world, but against Rayo Vallecano at Camp Nou, Rakitic could play the part. Luis Enrique never used this options this season.
From the reserves
He's the natural replacement. Barça B central defensive midfielder has played three matches this season with the first team and playing the 90 minutes. Twice in the Cup and once in the Champions League.
Positionally incredible and tactically perfect, Samper is capable of replacing Busquets during his injury.
Gumbau is Samper's replacement at Barça B. He can also play as a central midfielder. Gumbau has already made his debut with the first team in the match against Elche in the Cup (0-4). Luis Enrique gave him his debut and made him play 90 minutes.
Far from natural options. Luis Enrique could also try more original formulas. For example Gerard Pique played in this position when he played at Zaragoza. Xavi Hernandez could also play this part if needed.
In fact Xavi started his career playing in Busquet's current position and then changed. Sergi Roberto and Rafinha are options too, but it seems complicated that these last two could start as central defensive's midfielders during Busquets's absence.