Barcelona has lost its past two league games, allowing Valencia and Real Madrid to creep closer in the standings and inject life into the title race. Coach Frank Rijkaard's team has also recently been eliminated from the Copa del Rey.
Barcelona, with 52 points from 23 games, now leads Valencia by six points and Madrid by seven.
Striker Samuel Eto'o seemed unconcerned by his team's slump, saying its six-point advantage still represented a large gap.
"We're under no pressure because we can lose twice while other teams can't. If Valencia slips, up they'd be nine points behind. And another team (Madrid) 10. So who should be more worried?" said Eto'o, who was expected to be named Africa's player of the year for the third straight season in Abuja, Nigeria later Thursday.
The defending champion will be relieved to welcome back Brazil striker Ronaldinho, who was suspended for the defeats against Atletico Madrid and Valencia.
Ronaldinho, who helped Barcelona gain 46 points from 57 in his 19 league appearances this season, will be joined by midfielder Deco, who has recovered from injury.
Injured Ludovic Giuly and suspended Rafael Marquez will be sidelined, while Lionel Messi is likely to be saved for next Wednesday's Champions League game at Chelsea.
Betis coach Llorenc Serra Ferrer, who was in charge of Barcelona for most of the 2000-01 season, badly needs his 16th-place team to pull off a surprise after losing three of its last four games.
Valencia's 1-0 win over Barcelona last weekend has increased its belief it can win its third league title in five seasons.
Striker David Villa, who has scored the team's winning goals in the last two games, said Valencia will benefit from its lack of involvement in other competitions.
"We only have the league to concentrate on. I hope Barcelona and Madrid get affected by the Champions League and we stay on form," Villa told sports daily Marca.
On Sunday, Valencia aims for its fifth win in six games when it visits 13th-place Real Sociedad, which has won once and lost once under new coach Gonzalo Arconada.
Valencia goalkeeper Santiago Canizares, who has conceded just four goals in the team's 12-game unbeaten streak, will be making his 250th league appearance for the club.
Valencia continues to be without injured Patrick Kluivert, Vicente Rodriguez, Roberto Ayala and Edu. Forward Miguel Angel Angulo is suspended.
Madrid will feel mixed emotions when it hosts Alaves on Saturday.
The team was heartened by its rousing display which crushed Zaragoza 4-0 in the Copa del Rey semifinal, second leg on Tuesday. However, the victory wasn't enough to overturn a 6-1 deficit from the first leg.
Madrid's elimination left it with the Spanish league and the Champions League to target in its attempt to lift its first major trophy since 2003.
"After such an enormous effort, we want compensation. We have two competitions left and we have to keep going. The team is making spectacular progress. We have commitment, work, optimism and unity," Madrid coach Juan Ramon Lopez Caro said.
Madrid, which has won its last five league games, may be boosted by the inclusion of captain Raul Gonzalez in the squad for the first time since he suffered a serious knee injury in November.
Lopez Caro will lack two midfielders, suspended Thomas Gravesen and injured Pablo Garcia and, with Tuesday's Champions League game with Arsenal in mind, may rest Zinedine Zidane and Jonathan Woodgate.
Alaves, which looked a clear relegation favorite a month ago, has lost just once - to Barcelona - in its last five matches and risen to 15th position.
Last-place Mallorca will be under Gregorio Manzano for its visit to 18th-place Malaga. Manzano began his second spell in charge on Wednesday following the resignation of Hector Cuper.
In this weekend's other 24th-round games, it's: Cadiz vs. Osasuna; Deportivo La Coruna vs. Zaragoza; Sevilla vs. Celta Vigo; Villarreal vs. Espanyol; Racing Santander vs. Athletic Bilbao; and Getafe vs. Atletico Madrid.
