Sounds like sour grapes to me. There are several reports that he was moved on because his relationship with De Gea was too friendly, resulting in training sessions were too relaxed.
The Athletic wrote about it several times. From december: [
Source]
"Work on such aspects is done at Carrington, although
it is understood that concern over Emilio Alvarez’s attention to detail ultimately led to a parting of the ways with the goalkeeping coach in September. Alvarez’s close friendship with De Gea, which started when the pair worked together at Atletico Madrid, meant they often socialised together in Manchester and it was thought United’s No 1 was not being challenged enough in training.
Ole Gunnar Solskjaer brought in Richard Hartis to rediscover the best from De Gea and we have seen a number of his trademark saves. But aerial authority is an aspect that requires work."
- David Ornstein also commented
that the relationship was too friendly, and not work-focused enough.
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And upon a question about why De Gea was less consistent, United writer
Laurie Whitwell said: "Hi Rhys, as I mention in the piece, the lack of a truly competitive No 1 battle after so many years of high performance has been cited by people I've spoken to as possible reason for a subconscious easing off.
Allied to this was his relationship with former coach Emiliano Alvarez, which some felt was too friendly rather than work-focussed. Alvarez left this season."