Messi was forced off with a hamstring strain in Tuesday's Champions League win over Celtic and will be out for around six weeks.

The Argentinian has been at the heart of most of Barcelona's best attacking play this season, but Rijkaard insists his team can cope without the 20-year-old.

"He is out and he will be missed, but we are capable of winning without him and we have shown that," said the 45-year-old.

The Dutch coach also ruled out a change of formation for Sunday's game against third-placed Villarreal.

"We will keep playing in the same way, just with another player in his position," he said. "The team feels most comfortable with this way of playing, which we have used many times."

Rijkaard believes Messi needs to "get to know his body better" to avoid picking up such injuries in future.

"He must learn the correct way of working and get to know his body better. He needs to do all he can to avoid this happening again in future. It is a harsh lesson," he said.

Messi, who left the pitch in tears after picking up his injury on Tuesday, is likely to be replaced in the starting line-up by Thierry Henry.

Mexican forward Giovani Dos Santos returns to the squad and is another option for Rijkaard, who will be without the suspended Carles Puyol and the injured Rafael Marquez in defence.

The Barca coach has an otherwise full-strength squad to choose from for Sunday's match, as the Catalan outfit seek to close in on Real Madrid at the top of the Primera Liga.

Real beat Recreativo Huelva last weekend to pull five points clear of their arch rivals, who lost at Atletico Madrid.

However, Bernd Schuster's side have suffered an alarming dip in form since Christmas.

Since then, the champions have lost to Almeria, Real Betis and Getafe in the league.

Real also crashed out of the Copa del Rey after losing both legs of their last-16 tie with Mallorca back in January and this week found themselves eliminated from the Champions League after a disappointing defeat at home to Roma.

Despite all of that, Rijkaard claims Barca remain focused on their own performances.

"If we don't get the results we want, it will be worthless, so we need to play a good game (on Sunday) and see where that takes us," he said.

Villarreal, meanwhile, lie third in the Primera Liga.

Manuel Pellegrini's side are enjoying a wonderful season and sit just seven points adrift of Barcelona.

However, the team have been struggling to score of late and were held to a disappointing goalless draw at home to Osasuna last weekend.

French winger Robert Pires admits his side face a tough match on Sunday.

"It is a big pitch (at the Nou Camp) and the quality of Barcelona's players means you are always chasing the ball, so it's difficult to win there," he said.

Nevertheless, the former Arsenal midfielder believes his team are good enough to challenge at the top of the table.

"Our aim is to get into the Champions League. People only talk about Real Madrid and Barcelona, but if we are capable of winning on Sunday we can think about fighting for something more," he said.

Angel Lopez is suspended, while Fabricio Fuentes and Cani miss out through injury.

Giuseppe Rossi and Pascal Cygan are available again, though, after returning to fitness this week.