The 23-year-old has struggled for playing since Thomas Tuchel's arrival.
The Blues are reportedly interested in recruiting Erling Haaland this summer.
"There are players at 23 years old who are absolutely the finished product," Nevin, who made more than 200 appearances for Chelsea, tells football.london.
"They have the physique, they have that power. I honestly don't think Tammy has completely filled out yet. I think there is more to come and there will be a point when suddenly it happens.
"His natural ability is fine, his natural goalscoring ability is fine. It's all good enough for the top level. But if you're going to be a lone striker in the Premier League, you need that little something special.
"Olivier Giroud is world-class at holding the ball up and laying it off and he's the best in the world when it comes to attacking near the post. And Timo Werner is one of the quickest players you'll ever see.
"So Tammy has to bring something that is physically world-class to the team because her technical quality is good enough.
"Look at someone like Didier Drogba, did he explode at 18 years old, 19 years old? No, he didn't and that just shows that players mature at different speeds. Tammy will come back one summer and people will go, 'Who is that guy?' and that will make the difference. So I have no worries about Tammy being good enough for Chelsea."