Kenny, 31, has been banned for nine months by the Football Association and faces an uncertain future after traces of the banned substance ephedrine were found in urine samples taken in May.
The Republic of Ireland international told an FA hearing on Monday that he took tablets bought at a chemists to help with a chest infection prior to the Blades' play-off semi-final against
Preston, but although cleared of cheating was banned until April and will now miss out on his World Cup dream.
Crystal Palace boss Warnock, who plucked Kenny from non-league football when manager at Bury in 1998 and then brought him to Bramall Lane four years later, said: "It's a tragedy, he's a big, softy who's got no malice in him whatsoever.
"Anybody who knows Paddy knows he has just made an honest mistake, so it's a warning to other players, who must learn they can't just go into the local chemist and buy things like cold remedies.
"We had a similar problem with one of my lads this year, it's easily done, but it's so sad for Paddy because he's going to miss out on his World Cup dream."
Kenny, suspended by the Blades in June, is in the final year of his contract at Bramall Lane. He is understood to be considering an appeal, but if that fails will be available for only the last two games of the Coca-Cola Championship season.
Warnock added: "He's not going to play for the Republic of Ireland this season and he'll miss out if they go to South Africa. But it won't stop him getting a club because he's a good goalkeeper. He's the best in the Championship.
"He has nothing to do with drugs or anything like that and people who know him know that. That's why I'm sure Sheffield United will stick by him.
"In some respects his ban could have been two years. There doesn't seem to be any leeway with drugs, but there's no way he could have been allowed to go scot-free either.
"There are rules and he broke them, but it's such a shame for someone like Paddy."