Mikel Merino's lone strike at the Emirates handed Chelsea their eighth Premier League defeat of the campaign, exposing a lacklustre attacking display from Enzo Maresca's men.
Despite results elsewhere keeping the Blues in fourth place, their hopes of securing Champions League football remain under scrutiny. Sancho, on loan from Manchester United since the summer, struggled to make an impact against the Gunners, prompting fierce criticism from Gallas.
The winger, who shone at Borussia Dortmund with flair and dynamism, has managed just two goals and four assists in the Premier League this season, with his last contribution coming against Crystal Palace in early January.
Speaking on Stadium Astro, Gallas didn't hold back. "That player has talent, everybody knows this, but we don't know why he can't perform," he said. "At Dortmund, he was magnificent. At Manchester United, it didn't work. He came to Chelsea, and at first, we saw that Dortmund Sancho, but then he disappeared.
"He can't stay at Chelsea like this, he has to do more. In the Premier League, you must play at the highest level every game, not just turn up for a few and then fade."
Focusing on the Arsenal defeat, Gallas added: "Did you see him dribble past his left-back? No. He did nothing. He didn't challenge his opponent. He gets the ball, does some tricks, then passes it back. We want more from a winger."
Sancho's loan deal includes a conditional obligation for Chelsea to buy him permanently, though The Athletic's David Ornstein recently explained the Blues could opt out by paying a fee. "Technically, Sancho could return to United," Ornstein said, "but that wasn't the intention when this deal was struck. It'll be a big story if that changes."
The criticism doesn't stop with Gallas. Former Manchester United defender Paul Parker has also slated Sancho's form, branding him "a weak little boy" last month. "He should be looking in the mirror," Parker said.
"Chelsea must regret this deal, they'll be desperate to offload him, but finding a taker will be tough. He's brought this on himself."
As the Blues prepare for their next challenge, Sancho's future hangs in the balance. With his loan clock ticking, the 24-year-old must rediscover his Dortmund sparkâor face an uncertain summer.