Although stopping short of comparing the son of Dalakhani with his 2014 Doncaster hero Kingston Hill, the Newmarket handler feels he has every chance of succeeding in the final Classic of the season, for which he is as short as 5-1.
The three-year-old warmed up for his trip to Town Moor on September 16 when claiming his first Group Three success on Saturday in the Geoffrey Freer Stakes at Newbury, to make it four wins from as many starts this season.
Varian said: "I thought Defoe was very impressive at Newbury. I liked the way he moved into the race and he beat them fairly comfortably.
"I think we can be very excited about the St Leger. I have thought for a long time he could be a St Leger horse, even when he was winning those handicaps over 10 furlongs. It was encouraging to see him win over that trip as I always felt this horse would stay well.
"I don't think you can compare him with Kingston Hill, as they have gone on very different paths. Kingston Hill won a Group Three and Group One as a two-year-old and had run in the Guineas, Derby and Eclipse by the time he won the St Leger.
"This horse has come up through the ranks, winning two very good handicaps, a Listed race and now a Group Three. He looks a potential Group One horse, but he isn't yet as he has not run in one.
"Although they have taken very different paths, it doesn't mean to say that Defoe will not end up being as good. Both are similar looking horses that have got the ability and great attitudes to go with it."