Lineker has little doubt that Barça will lift the famous trophy at Wembley, even after a season in which United have secured a record 19th Premier League title.
"If Barcelona turn up, they win," Lineker told the BBC. "Barça have got to have a bad day for United to realistically have a chance. Either that or United get lucky."
Lineker admits United will benefit from the match being played at Wembley, but thinks any 'home' advantage will be overcome by a Barcelona side touted as one of the best in history.
"The fact it is in England and they will probably get the vast majority of the support and it is 90 minutes, not two legs, gives them a better chance," he added.
"But if they played ten times, United might win one or two of them, there would maybe be a couple of draws and Barça would win the rest."
He added: "Time will tell, but potentially they [Barça] could be the best club side ever.
"They are certainly the best side I've witnessed in terms of the attractiveness of their football, the way they have moved the game on."