He was told he had the disease some weeks ago and flew to America for treatment, which will last several weeks.
JP's racing manager Frank Berry confirmed: "He has been in America for treatment but he will be back home at the weekend and is in good form."
Punters were surprised when JP, a familiar face at races, was absent from the Grand National at Fairyhouse on Easter Monday, in which his ill-fated horse, Wichita Lineman, died.
The businessman and gambling king, who once won €1m from a bookie on a single afternoon, was due to fly home to Limerick today to be supported by his wife Noreen, family and friends.
A close friend of the man nicknamed the Sundance Kid said: "He is making a good recovery and is due back in Ireland today, and is looking forward to coming home and taking it easy for a while."
Last month, he attended the Cheltenham festival, where he seemed in fine form.
One friend of the keen golfer, who includes Tiger Woods among his friends, said JP was in good spirits.
"He is as enthusiastic as ever. This is just a small setback and JP is raring to go."
Sports presenter Bill O'Herlihy, who beat colon cancer last year, led the get well messages.
Bill (70) said: "I hope JP's cancer has been caught early.
"My cancer has been cured and I hope it turns out the same for JP. He will be in my prayers."
JP has vowed to press on with organising next year's JP McManus Invitational Pro-Am at Adare Manor, Co Limerick.
Tiger Woods and Padraig Harrington will tee up at the event.