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.
Bottas has highlighted race pace as a weakness, the Finn having finished 13th in the Miami event after 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 I think the fact is we definitely need those new bits that we're getting in Imola, so hopefully that will give us an improvement, and that way a better chance to fight for 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.
"We don't have any specific corners where we're struggling," he said. "If you look in Melbourne and Baku, we were struggling on the straights, but we struggle in the corners as well.
"It's everywhere, and that's the area where we're pushing the envelope in all areas. Not only with the car, but also with the operations, with the drivers.
"We've got a package for Imola. And hopefully, that will also help. The problem is that I'm sure everyone will have a package in Imola, so we'll have to see."
Pujolar noted that small margins can make a big difference in the midfield group.
"Everything is so tight that you have got some teams that struggle more," he said. "And then that's it, you are there again in the pack.
"So for us still, in terms of performance, we're not where we want to be and we're just pushing in all areas because it's very tight. And as soon as you get from track to track, there's always a reason that you can fall back."
