Dembélé was considered an attainable alternative to United's priority transfer target Jadon Sancho but the chances of agreement are now thought to be remote.
The protracted negotiations for Sancho reached an impasse weeks ago, with Borussia Dortmund unwilling to lower their €120m asking price.
United pursued other options and were interested in the prospect of taking Dembélé on loan, with the 23-year-old's place at the Camp Nou uncertain under newly-appointed head coach Ronald Koeman.
Barcelona are willing to part with Dembélé before the close of Monday’s transfer deadline but would prefer a permanent deal to help facilitate a move for Olympique Lyonnais' Memphis Depay.
Dembélé is also reported to prefer a permanent move to Old Trafford rather than a loan, but United are only prepared to sanction a temporary deal.