The Chilean contract rebel, who saw his bid to join Manchester City on deadline-day fall flat, was jeered by a section of his own supporters on Saturday.
But the striker turned those jeers to cheers with a stunning winner on his first start of the season.
Trailing early on against Cologne, Arsenal hit back in the second half through Sead Kolasinac and Sanchez to get Arsenal’s Europa League campaign off and running.
It had looked a different story earlier on. With the chaos involving ticketless fans forcing the game to be delayed by an hour, Cologne took full advantage of Arsenal’s dozy start.
And it was keeper David Ospina who was at fault, returning to his tradition of making a Euro clanger at home in September.
Two years ago in the Champions League, he dropped the ball over his own line to concede an own goal in the 3-2 home defeat by unfancied Olympiakos.
He did better last season, managing a clean sheet when Basel came to London and last 2-0.
But it did not take long for Ospina to make his howler.
It came in the ninth minute when the Colombian keeper - boss Arsene Wenger’s pick for Europe over Petr Cech - rushed off his line to make a sweeper-style clearance.
Only trouble was it went straight to Leonardo Bittencourt, bouncing off the midfielder to tee up striker Jhon Cordoba, who is also a Colombia international.
He took a touch, turned and then chipped from 35 yards over his back-pedalling countryman and Arsenal No.13 - proving that number was indeed unlucky - and into his unguarded net.
Cologne’s fans, already delirious at their first venture into Europe for 25 years, went ballistic, with flares being set off.
Their excitement was perhaps understandable because they have had an dismal start to the season in the Bundesliga, losing three from three and scoring just once.
How Arsenal ace Sanchez must have felt we can only guess, but he did come to the fore later with Arsenal’s second goal.
The day after Manchester City - who wanted to sign him last month - won 4-0 at Feyenoord in the Champions League, he return to the starting XI for this Europa League clash following an abdominal problem.
But he barely had a kick early on and the north Londoners’ early threat was very limited.
Alex Iwobi chopped through for Theo Walcott in the 23rd minute, but under pressure from Konstantin Rausch, the winger scuffed a weak shot well off target.
Ospina looked like he might have given a penalty away in the 31st minute, upending Jonas Hector as he dived in for the ball, but fortunately the flag had already gone up for offside. Four minutes later Hector had to hobble off.
Olivier Giroud failed to get enough on a header from an inviting Sanchez cross. At half-time boss Arsene Wenger hauled off Rob Holding, who’d had a shocker.
His replacement, Sead Kolasinac, needed just until the 49th minute to make his impact.
Walcott tried to cross from the left, but his ball flicked up off Lukas Klunter and fell nicely for the Bosnian to volley home a clean shot with his left foot.
With Arsenal having lost five of their last seven games at home to German opposition the goal must have come as a huge relief to boss Wenger.
He knew a third loss already this season would be a terrible preparation for the trip to champions Chelsea on Sunday.
The goal, though, did not lift the much-weakened Arsenal side to another level. And it was the away fans who continued to make all the noise, chanting with huge gusto and applauding any half-decent move by their team.
Hector Bellerin made sure of the result late on after scoring from Walcott's rebounded shot ten minutes from full-time.
