Reports surfaced on Thursday evening that the Chile forward had fallen foul of the Football Association’s ‘whereabouts’ rule by allegedly not being present for a routine doping test on Monday.
Sanchez was putting the final touches on his move to United, which saw Henrikh Mkhitaryan join Arsenal in an exchange deal, and an initial report from Spain highlighted the error.
Sanchez was still registered an Arsenal player when the apparent infraction occurred and Wenger conceded the club must take responsibility for the issue.
“It is just a bad day for him to be tested,” he said.
“Honestly, on the administration side it would still be our responsibility because he had not moved.
“The intention of Alexis was not to hide – and we have nothing to hide.
“On Monday there was a lot going on, it is a special day for Alexis Sanchez – you have do paperwork and travel.”
Here, Press Association Sport takes a look at Mourinho’s United reign.
Mourinho became the first United manager ever to mark his debut season at Old Trafford with a major trophy, claiming last season’s League Cup with victory over Southampton at Wembley.
United went on to add the Europa League with victory over Ajax, clinching Champions League qualification and making it a trophy hat-trick after having won the Community Shield at the start of the season.
Mourinho enjoys a 61.62 per cent win rate in all competitions since joining United, a total which compares favourably to Sir Alex Ferguson (59.67 per cent) and indeed tops all United managers.
United could only manage a sixth-placed finish in last season’s Premier League after 15 draws from their 38 games left them 24 points behind champions Chelsea. They have improved to second this time but still trail leaders Manchester City by 12 points.
His Premier League record since the start of last season reads 34 wins, 20 draws and just eight defeats, with 103 goals scored and 45 conceded for a goal difference of +58.
That League Cup triumph helped him build a 7-2 win-loss record in that competition with United, with four wins and one loss in the FA Cup.
The Europa League success contained 10 wins, three draws and two losses, with five wins and one defeat in this season’s Champions League group stage, while United won last season’s Community Shield against Leicester but lost to Real Madrid in this season’s UEFA Super Cup.
That leaves his overall record at 61 wins, 23 draws and 15 defeats as he prepares to bring up a century of games with United when they face Yeovil on Friday in the FA Cup.
Mourinho has spent reported fees of nearly £300m in the three transfer windows prior to this one since taking over at Old Trafford.
The £89million return of former United academy graduate Paul Pogba and the £75m signing of Romelu Lukaku headline his spending, though Alexis Sanchez arrived from Arsenal this month in a straight swap for Henrikh Mkhitaryan with no fee involved.
Mourinho has recouped around £57m, selling Morgan Schneiderlin and Memphis Depay for over £20m apiece, leaving his net spend at around £234m.
Jose Mourinho has signed a contract extension that will keep him at Manchester United until at least 2020, the club said Thursday, ending rumours he is unhappy at Old Trafford.
Mourinho became the first United manager in history to win a major trophy in his first season when he secured the League Cup last year, and he also led the club to victory in the Europa League, the only competition they had never won.
United are a distant second in the Premier League to Pep Guardiola’s Manchester City this season but are still in the FA Cup and the Champions League.
“I am really honoured and proud to be Manchester United manager,” Mourinho, 54, said in a club statement.
“I would like to say a big thank you to the owners and to (executive vice-chairman) Mr (Ed) Woodward for the recognition of my hard work and dedication. I am delighted they feel and trust that I am the right manager for this great club for the foreseeable future.
“We have set very high standards — winning three trophies in one season (including the Community Shield) — but those are the standards I expect my teams to aim for. We are creating the conditions for a brilliant and successful future for Manchester United.”
Mourinho, who was appointed in May 2016 on a three-year deal, replaced Louis van Gaal with a brief to return the club to the pinnacle of the English game.
The 20-time league champions have not won the Premier League title since Alex Ferguson’s last season in charge in 2012-13.
“I feel privileged to work with such a fantastic group of boys.”
— Manchester United (@ManUtd) January 25, 2018