Isak scored a first goal in the Premier League for Liverpool in Saturday's win against Crystal Palace.
But Carragher told the Liverpool Echo: "(Big spending) hasn't worked.
"It might work in the future because I don't think you can judge every signing in the first year.
"But I've said this before; my experience as a Liverpool player, every player who came to the club, I can't remember one who didn't really do much in his first season, and then was a revelation afterwards. Isak hasn't got going. He obviously got the injury, but he didn't look great to start with."
Carragher added: "In terms of what Liverpool need to do next season, well obviously they need to replace (Mohamed) Salah.
"I think for me, three players need to come straight into the team.
"You can't go on and buy five players into the team. Liverpool bought a lot of players last summer, so they are not going to buy a load this summer."