There's a noted Biblical story, I believe from the Book of Acts in the New Testament. It in a way highlights Alexis Sanchez's form at Manchester United, especially in contrast with his form at Arsenal.
For those not familiar with the story, Saul was a bad man and a persecutor of Christians, when on a journey to Damascus, he encountered Jesus and sought to change his ways. It's where the term "Damascene conversion" comes from. It denotes a rapid and radical change a person has from one way to another.
Alexis since joining United in my view has had a reverse Damascene conversion. Saul turned from bad Saul to good Paul. Alexis has gone from good Arsenal Alexis to not so good United Alexis.
Is there a reason for this change? I'm not a Manchester United fan, but I saw the game vs. Leicester City.
He had a few chances, but his distribution and pace were poor. This is a stark contrast to his Arsenal form, where his pervasive energy made him arguably the best wide forward in the league.
This video, showing his game ironically versus United in the 2015/16 season, denoted his true qualities. Another top game he had for Arsenal was against West Ham in 16/17, and the 2017 FA Cup Final win vs. Chelsea. His all-energy game were crucial factors Arsenal's win in all three aforecited matches.
Is it the difference between Wenger and Mourinho's styles? Is his place and role in the respective teams different? Are there other issues beyond football causing this?
And what happens if he continues to be bad? Alexis had pedigree prior to playing for Arsenal, as he joined them from Barcelona. And at Barca, he combined well with the great Messi, Xavia and Iniesta.
The most recent game at this point was versus Wolves, and again he didn't perform.
It is time for him to be dropped?
Could it be the fact Chile didn't qualify for the Russia World Cup earlier this year?
There could be a multitude of factors triggering this dip in form. Alexis was a top player at Arsenal, and to use cliche players don't turn bad overnight. There may not be a discernible reason. It's interesting however to note why this is the case, and not ponder if we're seeing the end of one of the great players of recent times.
#4 Different Playing Style
Arsene Wenger, his manager at Arsenal, of course, emphasised free-flowing attacking football, and players employing their own intelligence and initiative to handle on-field decisions. His role as manager was then to prepare players physically and mentally, and ensure they could play their highest technical game.
As a player who is high energy, highly technical, and self-driven, this environment suited Alexis initially. Hence why he scored many goals for Arsenal in his tenure.
Mourinho, however, has a near opposite style for Wenger. He's more pragmatic and likes to play football best suited the opponents at hand. He can play "sexy football" when needed, but it's not primary as it would have been for Wenger. Or for Guardiola or Klopp as of now. Mourinho also is a manager who likes to control tactics and structure much more than Wenger did, so it's possible that this new style isn't suiting Sanchez at all.
This is speculation of course, but it can take time for players to adjust to new styles and formats.
#3 Not the main man
From 2014 to 2018, Arsenal certainly had many other good players - and like Alexis himself, some world-class talent. Giroud, Koscielny, Ozil, Cech, Ramsey, Lacazette, and Mertesacker were all players who had achieved some notable points in their careers.
However, Sanchez was Arsenal's major goal threat and their best player overall.
His man of the match display in the 2017 FA Cup Final win vs. Chelsea was a testament to this, and it was ironically this focus as the main man that triggered his departure.
At United, he is one of several other top players, and maybe the manner in which the team is constructed (and Mourinho's different managerial style vis a vis Wenger) restricts his ability to be the best on the field.
Given Sanchez's goal output, he may have had a near venerated status at Arsenal, in acknowledgement of this contribution. At United, and with Lukaku as the main goal threat, and Pogba as a recent World Cup winner, he may not be as critical to United's success. As not being the "main man", he may not have as much a drive to succeed or push himself.
#2 Personal Issues
Alexis recently noted he is splitting from his long-term partner Mayte Rodriguez. Players are human, and they have issues in their lives that can affect their performances like any other.
Moreover, maybe he doesn't like the north-west of England? Manchester is a fine city - but it doesn't compare to London. To be fair, very cities on Earth do.
But there could be a number of reasons why he hasn't performed well yet, which transcend the footballing scope.
Chile also didn't qualify for the 2018 World Cup, which was a shock given their recent successes in the Copa America. Alexis for the large part was a key factor in these wins, scoring the penalty that won them the competition in 2015:
As part of a golden generation for the country, maybe not qualifying for the World Cup was a blow that was hard to recover from.
Professional sportspeople should learn to handle defeat. But then they are still human, and defeats can still affect them.
#1 Contrast
Sanchez's most recent game for United was the 1-1 draw at home to Wolverhampton Wanderers. And again, his performance referenced this stark contrast. People can speculate what the exact cause of it is.
But in a way, it's sad to see what has been one of the Premier League's top players in recent times to descend in his quality and output.
Can he regain it? Yes. But he and Mourinho need to do much soul-searching to recover his form.
Could he go back to Arsenal? Wenger has gone, and it's evident that the attitude under him will evaporate too, now that Emery is the head coach. A player of his style could do well under Emery's pressing game, and Arsenal does need a wide forward.
OK, that's fanciful, but talk of him leaving at this point is moot at best. He needs more time to bed in, but for United and his sakes, he needs to recover the Arsenal form.
Arguably Sanchez's best ever performance in England was, ironically, against Man United. It would be good if only based on his apparent talent, for him to re-capture this.