Barcelona head coach Ronald Koeman has called on Antoine Griezmann to step up this season and help fill the void left by Lionel Messi.

The Catalan giants are still coming to terms with the loss of their all-time leading scorer, who officially joined Paris Saint-Germain on a free transfer this week.

Six-time Ballon d'Or winner Messi scored 672 goals and assisted 265 more for Barca across 778 appearances in his 17 years in the first team.

He directly contributed to 39 goals in LaLiga last season, scoring 30 and setting up a further nine.

Griezmann was the only other Barca player to reach double figures for goal involvements (20), with Ousmane Dembele next on the list with nine.

As they prepare to begin a first season without Messi since 2003-04, Koeman has challenged World Cup winner Griezmann to improve on those figures.

"I love working with him," Koeman said at a news conference ahead of Sunday's opening fixture against Real Sociedad. "He is a team player who always helps without the ball.

"He is always happy and encourages others. But he must take another step and be more effective. It will be more important without Leo, because he occupied that position."

Messi's departure came after Barca had agreed deals with Memphis Depay and Sergio Aguero, the pair arriving on free transfers from Lyon and Manchester City respectively.

Aguero did not feature for Barca in pre-season and is out for another 10 weeks through injury, but Depay has made a positive impression with three goals in four friendlies.

"I know Memphis well from our time with the Netherlands national team," Koeman said. "He has been effective with his performances so far.

"We have lost a lot with Leo leaving and now we ask for more from the others, and not just the forwards. We will try our best to improve and achieve our targets."

Barca received a big boost on the eve of their season opener with Sociedad as it was confirmed Depay, Eric Garcia and Rey Manaj have been officially registered to play.

Ansu Fati is out until after the September international break, however, while Ousmane Dembele and Aguero are also nursing injuries, leaving Koeman light in attack.

Barca's well-documented financial problems make bringing in new players difficult, but Koeman is hopeful of another new striker arriving before the transfer window closes.

"We lack players, and some are still injured," he said. "The squad is short. That's why we need a striker. 

"But then it depends on whether we can sign the player. It depends on the Financial Fair Play issues. If we can, we will sign someone."

Sunday's clash with Sociedad will mark the first time Barcelona have played a competitive game in front of fans at Camp Nou in 17 months, with up to 30,000 expected to be in attendance.

It will be an emotional occasion in more ways than one as Barca begin a new chapter without the greatest player in their history.

"Messi's exit was painful for everyone because of everything he has done," Koeman said. "We have to live the today and not the past. I am excited and hopefully the fans are too.

"I am excited to play in front of our fans, though we know the quality of the opponent, who we played against many times last year."

Barca are unbeaten in their past 10 league games against Real Sociedad in LaLiga (W8 D2). Only once have they gone 11 league matches without defeat in this fixture, a run of 17 games between 1952 and 1960.