The 64-year-old was in Silkstream Park, near the A5, when he had been allegedly attacked on Thursday (January 16) at 9.17pm.
He was taken to hospital for treatment, where he remains.
Part of the park was closed off as a cordon was in place.
A Metropolitan Police spokesperson said: “Officers responded to an alleged assault on a 64-year-old man in Silkstream Park, at 9.17pm on Thursday (January 16).
“London Ambulance Services also attended the scene to treat the victim. He was later taken to hospital for treatment where he remains.
“A crime scene remains in place while police investigate. Officers are calling for witnesses to come forward with information – calling 101, quoting 7145/16JAN. No arrests have been made at this stage.”
