The Catalan giants will be without Ansu Fati for the rest of the year after the teenager picked up a serious injury last weekend.
While Lionel Messi seems to back among the goals, Antoine Griezmann still needs to step up but Rivaldo feels the club needs to dip into the transfer market.
"I thought Barcelona should sign someone before Ansu Fati's injury, so now I think it even more," the ex-Brazil star told Betfair.
"The club should try to sign one or two strikers if they want to fight for the Champions League.
"Koeman should have more options in attack to manage a line-up which is sufficiently decisive and competitive in big games, in important ties.
"Ansu was playing well, he was gaining confidence and becoming more influential in the team, his absence is another headache for the coach."