Grant paid Bolton £15m for the French striker nearly four years ago.
The 32-year-old is moving to Chinese club Shanghai Shenhua in a £175,000-a-week deal. He won the Double with the Blues but was frozen out by boss Andre Villas-Boas.
Grant said: "He is the most gifted player I have seen - he is quick, can beat players and can score.
"On the quality side he is unbelievable, but he (had) the lack of passion to succeed and to push himself to the end.
"He did well for Chelsea but could do better."