Australia captain Michael Clarke has said the retiring Indian legend Sachin Tendulkar has been the "greatest constant" in world cricket for a generation and the best batsman of all time after Don Bradman. Paying tribute to the 40-year-old, who has been an international cricketer for 24 years, Clarke said he can't really believe that the Indian veteran has grown old. Clarke wrote in his column for 'Daily Telegraph' that he had been lucky enough to play with and against some great batsmen but there have been none better than Sachin in his time.Tendulkar is retiring from international cricket at the end of his 200th Test, which is currently being played in his home city of Mumbai against the West Indies.