The 17-year-old striker recently underwent his third trial with the Red Devils following two previous experiences at the Carrington academy.
Reports last week indicated that United were set to make a £150,000 bid for the Scotland U17 international but a source has told Tribalfootball that the third trial only came about because the player was injured during his second spell.
It is understood that United simply wanted to give Knox another chance to prove himself, and there will be no offer forthcoming for the talented youngster.
"Knox is only back at United because he got inured last trial," said the source.
"They will never sign him."
Last season, Livingston turned down an offer from Scottish giants Rangers for Knox because it did not meet their hefty valuation, and his form in the current campaign indicates perhaps he is not yet ready to move to a club such as United.
"No one is going to pay the money they (Livi) want," the source added.
"Especially how he's now a year older and not getting a game in a league below."
Knox played seven times in the Scottish Championship last term but has only featured on thee occasions in League One under David Hopkin.
Knox told the Daily Record earlier this month that he was waiting to see what comes of his time spent with Nicky Butt's U23 side at United.