The Scotsman ended his campaign with an Olympic gold medal and his first Grand Slam title, which he gained at the US Open after defeating world number one Novak Djokovic in the final.
"I think [he can achieve] a lot more," Lendl told BBC Sport. "I'm not going to say number of Grand Slams, I'm just going to say where Andy is now.
"If take 'point A', when we started working, now he's at 'point B' and when I envisage I would say he's about 20% there."
Murray ended the season with a semi-final defeat to Roger Federer at the ATP World Tour Finals in London.