Griezmann has struggled to hit top form in his first season at Barca, scoring 14 goals in 37 games since his €120million arrival from Atletico Madrid.
Barca are heavily linked with a move for Inter star Lautaro Martinez, who is considered a long-term replacement for Luis Suarez.
Former Arsenal manager Wenger believes Griezmann will improve, but feels Barcelona will add another forward this close season.
"Barcelona has always bought the greatest players. Personally, I'm not sure that Suarez needs to be replaced," he said.
"They can add another striker because it's true that Suarez and [Lionel] Messi are the main contributors and they might need somebody else who can score goals.
"I still think Griezmann at some stage will start to score goals, he doesn't look completely adapted, maybe now after confinement he will be refreshed because he's a good goalscorer as well.
"But I'm convinced they will buy another striker as well."
While Messi has scored 24 goals this season, Suarez has – like Griezmann – contributed 14, although he has played in 14 fewer games.
Barcelona, two points clear atop LaLiga, resume their campaign away to Mallorca on Saturday.