The Professional Footballers' Association chief executive Gordon Taylor told ITV News there were unreported cases all over the country after six former players came forward in the past week.
It comes as the FA announced Sunday morning it has instructed a senior lawyer to help it investigate historical sex abuse claims.
Kate Gallafent QC will oversee the internal review and a "wide-ranging inquiry may be required in time", the FA added.
"Perhaps in the light of of what has been revealed in others areas at that time we should not be surprised but we shall wait to see how many players do come out," said Mr Taylor.
Andy Woodward, a former player for Crewe Alexandra, Bury and Sheffield United, was the first to reveal he had been abused by youth coach Barry Bennell.
After waiving his anonymity, he said: "We’ve seen with the Jimmy Savile case how people have had the courage [to speak out], yet I’d say within the football world it’s even harder.
“Only now, at the age of 43, I feel I can actually live without that secret and that massive, horrible burden.”
Bennell, who worked for Crewe, Manchester City, Stoke and junior teams in north-west England and the Midlands, abused young boys from the 1970s onwards.
He was jailed for four years for raping a British boy on a 1994 football tour of Florida, and given a nine-year sentence for 23 offences against six boys in England in 1998.
He was jailed for a third time in 2015 after admitting abusing a boy at a 1980 football camp in Macclesfield.
Jason Dunford, a former Manchester City youth player, claims he was one of many young victims.
Speaking to ITV News, he said "I think if I found out that there's over a thousand people (who've been abused as young footballers) it wouldn't shock me, (but) it's going to shock the public."