The Hinwil team had a new front wing in Australia and a revised beam wing in Miami, but Bottas says that more is needed in order to move the C43 further up the grid.
The Finn has highlighted race pace as a weakness, with the former Mercedes driver having finished 13th in the Miami event despite making Q3.
"For some reason on a single lap on the soft tyre, we were strong, but then in the long run, I think that's where we are," said Bottas. "I couldn't do much more.
"The start was good and we were in a good position. I tried to hold on to the top 10, but unfortunately, I think this reflects more the true pace of the car.
"Definitely good knowledge, but the fact is we definitely need those new bits that we're getting in Imola. Hopefully that will give us an improvement, and that way a better chance to fight for the top 10.
"We do have quite a bit of new stuff for that. So looking forward. We definitely need it, so yeah, let's see how we go there."
Alfa Romeo head of track engineering Xevi Pujolar admitted that the team has to raise its game in all areas.
Source: Autosport