Mourinho is widely expected to take over and Express Sport understands an announcement could be made as early as tomorrow.
Van Gaal only guided United to FA Cup glory on Saturday, before finally being sacked this morning after meetings at the club's Carrington training ground.
But according to The Mirror, Van Gaal had known he was heading towards the exit after Mourinho had made a call to him a few weeks ago.
The Portuguese tactician is said to have phoned Van Gaal and told him that the club had sounded him out as his replacement.
Van Gaal had been confident he would see out the final year of his contract until Mourinho delivered the bad news.
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