He's already one of the best midfielders in the world and still has significant room for improvement," said Letang.
The French club has not named a specific valuation but referenced recent high-profile Premier League transfers as benchmarks for the teenager's market value.
"You have to look at his level.
How much have players like [Elliot] Anderson or [Sandro] Tonali been sold for, who are much older, but without room for improvement, so you have an idea of the value of Ayyoub, who has a unique profile at only 18-years-old," explained Letang.
Beyond midfield reinforcements, Manchester United is looking to strengthen its attacking options on the left flank.
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Aston Villa winger Morgan Rogers has emerged as a primary target with a reported valuation of £130 million, while Everton's Illiman Ndiaye and Lille's Matias Fernandez-Pardo remain under consideration.
