It seemed he had pulled off an amazing last-ditch coup.
But Press Association Sport understands the paperwork was not completed in time and Coentrao will remain at the Santiago Bernabeu.
The news will be greeted with dismay by United fans, who have been less than impressed with the club's dealings during the window.
And it leaves Moyes in the tricky position of needing to explain to Patrice Evra why he felt another left-back was required.
One theory was that Coentrao would have operated at left-back, with Evra in a more advanced role, which he played during his early years as a professional.
Nevertheless, Evra's form last season and the early weeks of the current one was not that of a man who needed replacing.
Now the France star will remain in his favoured role, with Alex Buttner also set to stay at Old Trafford as his deputy following talks over loan deals with both Besiktas and Celtic.
- Sports & Recreation
- Patrice Evra
- David Moyes