United have long been linked with a move for the Barcelona star despite him only recently penning a new long-term deal with the club in October.
The Red Devils would likely have to trigger the Brazilian’s £178million release clause making any move the richest in history by some distance.
And Mourinho has distanced himself or his club from such a deal describing it as akin to trying to break into a safe.
“I always try to be objective and pragmatic with the clubs and ask the clubs for what can be given to me. Neymar is absurd,” he told ESPN.
“A club like Barcelona cannot and will not lose Neymar.
“Although Messi is still a young player with years to go, he is already 30, and Neymar will be the great player of Barcelona after Messi.
“I think (trying to sign him is like) going to try to break into a safe. Impossible.”
Neymar recently hinted a move to the Premier League and possibly linking up with Mourinho could be in his future despite the clear obstacles in place.
"The Premier League is a championship that amazes me,” he said. "I like the playing style and the teams. And who knows, someday, I would like to play at it, yes.
"I admire Manchester United, Chelsea, Arsenal, Liverpool - these are the teams that are always there fighting.
"And then you have high-level coaches like (Jose) Mourinho and (Pep) Guardiola. These are coaches with whom any player would like to work."