The striker is currently away training with Chile ahead of their games against Costa Rica and Honduras where he is said to be "acting weird".
Clearly his club frustrations have filtered into the international break, with Sanchez cutting an isolated figure in training.
The 29-year-old was only named as a substitute for the 3-1 Manchester derby defeat to City at the weekend.
SunSport understands Sanchez was "furious" at not making the starting XI.
After the defeat, the former Arsenal star "slammed his boots down in the changing room" and was openly critical of the state of the defending.
He was said to have been left frustrated with the defensive tactics employed.
His mood has not brightened even after flying out of Manchester for international duty.
La Trecara reports that Sanchez was "silent for over an hour," sat on one of the training pitches on his own.
His team-mates were nearby playing football tennis.
SANCHEZ'S REAL DEAL
ALEXIS SANCHEZ has been "offered to Real Madrid" as he desperately touts himself around for a move away from Manchester United, according to reports in Spain.
Los Blancos are in the market for another forward with the former Barcelona star potentially on the move.
Sanchez, 29, revealed he feels alone at Old Trafford earlier this week and still struggles with the language barrier in England, despite his time at Arsenal.
David De Gea, Juan Mata and Romelu Lukaku are thought to be Sanchez's only close pals at United – and the Chilean is now leaning towards a January move.
He was eventually persuaded to join in with the other players, but his behaviour was described as strange by some of the Chile stars.
The Times claimed earlier this week Sanchez feels "lonely" and "ignored" – as he struggles to make any impact on the pitch for United.
Sanchez wasn't even Mourinho's No1 choice, with the Portuguese instead hoping to land long-term target Ivan Perisic from Inter Milan.
A source close to the Chilean recently revealed the player has had enough at United.
The source said: “He is totally fed up and at the end of his tether with Mourinho. He knows he can’t leave right now but is in talks with his agent about what they can do to try and sort the situation out.”
Alexis Sanchez's horror United Prem stats
ARSENAL
Games – 122
Goals – 60
Goals per game – 0.49
MAN UNITED –
Games – 21
Goals – 3
Goals per game – 0.14
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