The Manchester United boss confirmed on Wednesday he will retire at the end of the season, bringing to an end an incredible trophy-laden career.
But Shiels expects football bosses to continue to look up to a manager whose achievements are unlikely to be matched.
He said: "Everybody takes inspiration from someone - that's what keeps us living.
"He is one of the people you look up to, especially with what he has achieved and what he has done for such a fantastic club.
"He's a unique person. There's no such thing as being irreplaceable - but he probably is.
"No-one will manage a club of that magnitude for 26 years. No-one will do that again because of the way football has gone."