The team held fifth place earlier in the season before being overhauled by McLaren, but Sauber has gained 37 points on it in the last four races to close the gap to just 17.
"The are strong and going a good job, in a way they are where we were at the beginning of the year and have now taken our mantle so it is a cause for concern," Fernley told AUTOSPORT.
"In terms of the financial side, it is quite significant.
"Obviously we were in seventh last year and if Sauber did beat us we'd be seventh again so it's no different to last year, excepting that we assumed we would get a minimum of sixth.
"From an objective point of view, it's not achieving the targets we set ourselves."
Fernley believes that its hopes of retaining sixth are dependent on other teams taking points off Sauber as the Force India is not competitive enough to beat the Swiss squad.
Force India has managed only three points in the last seven races, meaning that at the current rate it is set to be overhauled.
"We simply don't have the pace, we are just hanging around at the back end of the top 10 and the Saubers are looking strong so we can't challenge them," said Fernley.
"We probably did better than we thought we would do [in Suzuka] so that's a good sign.
"We need to take another little step and that might help us, it does not need to be a huge amount.
"But both the race and the qualifying pace of the Sauber is such that unfortunately we can't compete with it."