The European champions, who have a 100 percent record in Group Five after six games and a six-point lead over closest rivals Bosnia, host fourth-placed Belgium in La Coruna on Saturday and fifth-placed Estonia in Merida on Wednesday.
Spain were vulnerable to the counter-attack in their surprise 2-0 defeat by the United States in the Confederations Cup in June and had to come from two down to beat Macedonia 3-2 in a friendly last month.
Brazilian-born Senna, 33, missed both games due to a hamstring injury, having been one of two midfield holding players at Euro 2008 alongside Xavi.
"He is a very important player in footballing terms and on a personal level," Capdevila told a news conference.
"He is a great colleague and gives us more balance in defence. He had an injury, unfortunately, but has overcome it and is getting back to his best. He is as vital here as he is with Villarreal."
Strike duo David Villa and Fernando Torres are expected to lead the line while Cesc Fabregas could join Xavi, Senna and David Silva in midfield.
BELGIUM WORRIES
Coach Vicente del Bosque is spoilt for choice with Santi Cazorla, Albert Riera, Xabi Alonso and Juan Mata also pushing for places, in stark contrast with Belgian counterpart Frank Vercauteren.
Belgium's caretaker boss has a number of injury worries as he looks to break a run of three consecutive qualifying defeats.
Vercauteren will have to wait until the last minute before naming his team.
Goalkeeper Stijn Stijnen is likely to miss the game against Spain but recover in time from a hip injury to face Armenia four days later.
Doubts over the fitness of second-choice Silvio Proto have also forced Vercauteren to call up a third keeper in Yves De Winter.
Defender Jelle Van Damme is also doubtful with a leg injury while striker Moussa Dembele, who missed training on Wednesday with flu, will undergo a fitness test on Friday.
Probable teams:
Spain: Iker Casillas; Alvaro Arbeloa, Gerard Pique, Carles Puyol, Joan Capdevila; Cesc Fabregas, Marcos Senna, Xavi, David Silva; Fernando Torres, David Villa
Belgium: Silvio Proto; Timmy Simons, Vincent Kompany, Thomas Vermaelen, Gill Swerts; Steven Defour, Axel Witsel, Kevin Mirallas, Marouane Fellaini; Moussa Dembele, Wesley Sonck.
‘The pure simplicity of the way Slot has managed the squad is probably the biggest thing I could say about him. It’s not broken, so let’s get on with it’: Liverpool legend full of admiration for Jurgen Klopp's successor at Anfield
‘I trained at Spurs and thought they’d give me a chance. But I received a letter thanking me but saying they didn’t think I was good enough – I was gutted’: How Tottenham missed out on signing Wales legend John Toshack
