Barca have missed the dynamic Spain international, who has been sidelined for three matches with a thigh strain, and his return to the squad will help boost damaged morale.
The constant praise heaped on the team after their sparkling start to the season is behind their dip in form, said Barca sports director Txiki Begiristain.
The Catalans were 12 points clear in the Primera Liga three weeks ago but a 1-1 draw at Olympique Lyon in the Champions League, another draw and two consecutive defeats in the domestic league have burst the aura of invincibility that surrounded Pep Guardiola's side.
"When people keep telling you, you are so good and that you are the best in the world it is difficult to maintain the same level of tension," Begiristain told Catalunya Radio.
Champions Real Madrid have closed the gap at the top to four points on the back of a 10-match winning streak.
"The players are still generating a lot of scoring opportunities and this is very positive," added Begiristain. "But it is fundamental we stress the spirit of sacrifice needed to recover the defensive solidity we have lost.
NO TIREDNESS
"The first thing everyone should remember is their individual responsibilities. It isn't down to tiredness."
Barca's success meant they played two games a week through January and into February unlike Real Madrid, who are out of the King's Cup.
Guardiola's team hold a 2-0 lead from the first leg against Mallorca.
Brazil striker Luis Fabiano was a surprise inclusion in Sevilla's squad for the other semi-final second leg at Athletic Bilbao after last season's top scorer had been out with a knee injury.
Sevilla president Jose Maria del Nido was in confident mood for the match with the 'Lions'.
"We are going to eat the Lion from its mane to its tail," he told reporters.
Athletic, who trail 2-1 from the first leg, welcome back leading scorer Fernando Llorente after he was rested for the weekend league encounter between the sides which Sevilla also won 2-1.
