Barcelona has defensive worries ahead of Tuesday's showdown with Arsenal which midfielder Seydou Keita has described as crucial for its season.

Keita scored the only goal as Barcelona kept up the pressure at the top of La Liga with a 1-0 win over Zaragoza at the weekend and now it turns its attention to the Champions League where it must overturn a 2-1 defeat at the Emirates to progress to the quarterfinals.

"Against Arsenal we have to go out with a strong mentality as this is probably the most important game of the season," said Keita.

"The key is not to let them score a goal because I'm sure that (Leo) Messi, (David) Villa and Pedro (Rodriguez) will get on the scoresheet in attack for us."

The bad news though for the Catalan side is that both its first-choice centerhalves will be missing with Gerard Pique suspended and Carles Puyol injured.

Puyol had hoped to be ready in time after a knee injury he suffered in mid-January but following a training session on Sunday he was ruled out.

His commanding presence at the back will be a big loss as will Pique's ability to bring the ball out of defense and open up the opposition with his range of passing.

Against Zaragoza, a make-shift side was put out by coach Pep Guardiola as he looked to rest players ahead of Tuesday's game.

Keeper Victor Valdes returned from injury but Eric Abidal, Andres Iniesta, Sergio Busquets and David Villa were all on the bench.

Abidal has been in excellent form in recent weeks and he is expected to move from full-back into the center alongside the injury prone Gabi Milito, whose Barca career has been blighted by long spells in the treatment room.

The rest of the team though should be at full strength and it will be an ominous task for the Arsenal defense as it faces a forward line which has already scored 78 goals domestically.

It is arguably a stronger proposition than last season when the teams met in the quarterfinals of the competition.

Arsenal drew that time at home but in the return leg it was taken apart 4-1 thanks to a majestic performance from Messi who scored all four goals.

Since then Barca has off-loaded Zlatan Ibrahimovic and brought in Villa who has fitted in seamlessly into the side's quick, two-touch attacking football.

It is true that Arsenal has also improved and will have learnt from its experience last season when it tried to play Barca at its own game and came off a clear second best.

Arsenal's second place in the Premiership is clear indication of its development with Jack Wilshire having emerged, among the exciting talent, in midfield alongside Cesc Fabregas.

While Wilshire has described himself fit after picking up a knock on his ankle in the 0-0 draw with Sunderland on Saturday, Arsenal too has important injuries with Theo Walcott and Robin van Persie out while Fabregas and Alex Song are doubts but expected to be fit.

Keita, who is likely to be on the bench for Barca with Xavi, Iniesta and Busquets making up the midfield, expects a tougher clash this time round.

"It will be a different game because Arsenal will come here with more experience and with new ideas of how to face us. We are though playing well in the La Liga and that gives you confidence for the other competitions," he said.