The 36-year-old man was injured at about 6.05pm last night.
He is in a stable condition in hospital and no arrests have been made yet.
The offender is believed to be a white man, about 5ft 9ins tall with a slim build, light brown hair and a beard.
He was wearing a white top with white sleeves and blue trousers. A man matching this description was seen walking from the scene shortly after the victim was found.
A scene watch was put in place by police.
Investigating officer Detective Inspector Will Crowther, based at Slough police station, has asked people to ‘avoid speculating’ on what happened as a full investigation was being carried out.
He said: “I am also aware that there has been footage relating to this incident shared on social media. I would ask people not to do that out of respect for the victim, and instead to contact the police if you have any footage which could be relevant to our investigation.
“If you have any details which you think could be helpful to us, please call the Thames Valley Police non-emergency number on 101, quoting reference number 43200369754, or call Crimestoppers anonymously on 0800 555 111. Alternatively, you can make a report online.”