A monitoring system for a gearbox containing a lubricant is disclosed. The monitoring system may include a first sensor configured to measure a concentration of debris particles in the lubricant, a second sensor configured to generate data indicative of a deterioration of the gearbox, and a controller in communication with the first and second sensors. The controller may be configured to estimate a concentration of debris particles in the lubricant based on the data from the second sensor using a mathematical function, and estimate a remaining useful life of the gearbox based on a difference between concentration of debris particles measured by the first sensor and concentration of debris particles estimated to be in the lubricant.