Adam Johnson has "damaged the reputation of football" and his chances of playing again are "very remote", says Professional Footballers' Association chairman Gordon Taylor.
Ex-Sunderland player Johnson has been jailed for six years for grooming and sexual activity with a girl aged 15.
Taylor also said the game needs to work harder on child protection.
"I do know my members and this is an example of something going badly wrong," Taylor told BBC Sport.
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Johnson, 28, began his career at Middlesbrough before moving to Manchester City in 2010 and then on to Sunderland two years later.
He also played 12 times for England.
Taylor said young women, "particularly girls", are a danger zone for footballers, who need to know there is a line they cannot cross.
"It's hoped that every industry will be even more alert after what has happened because these cases illustrate what can go wrong," he added.
"It's not for me to say whether six years is fair. That's laid down for the judge.
"I note the comments of the judge as well and, from that point of view, I'm not surprised at that sentence."
There were reports that Johnson would be stripped of his England caps following his jail sentence, but BBC Sport understands that is not the case.
Meanwhile, the NSPCC has written an open letter to Football Association chairman Greg Dyke saying it is worried football clubs are not taking the issue of child protection as seriously as they should.
The letter, signed by NSPCC chief executive Peter Wanless, said the organisation was worried football had a "cultural problem", adding: "This is not only about one rogue player that behaved badly, but a club that seemingly did not have child protection priorities embedded into their culture.
"It was not equipped to handle these allegations and seemingly did not deal with them appropriately, or indeed seriously. Furthermore, it is concerning if this is being forgotten by a club at the top level of football."
The NSPCC said it "would like to work with the FA to hammer home the message that this kind of behaviour should not be tolerated at any level and certainly not brushed under the carpet".