Top scorer Morata returned to the Blues' starting lineup for the 3-3 draw against Roma on Wednesday following a short layoff through injury, linking up with Hazard for the opening goal of the night.
Willian believes that, while both players are near the top of their respective games, they still have the capability to move up another gear to help Chelsea in their pursuit of silverware.
"They are great players and have great quality," he told the London Evening Standard. "But they can improve more. I think now we must improve and to try and win the other games."
Morata has netted seven goals in nine matches since arriving at Stamford Bridge in the summer, while Hazard has bagged two in the same number of outings.