Mikel Arteta spoke about the injury afterwards

LONDON—Mikel Arteta did not sugar-coat the moment Gabriel Magalhães signalled to come off. “I don’t know exactly what it is,” the Arsenal manager admitted after the shock FA Cup exit, “but when a player is asking to be substituted, it’s never good news.”

Gabriel felt something in his thigh late Saturday and left the pitch clutching the back of his leg. Arsenal will scan the Brazilian in the next 48 hours; his availability for Tuesday’s Champions League quarter-final first leg against Sporting CP is already in doubt.

Arteta’s concern is understandable. Last spring William Saliba’s late-season injury contributed to a collapse that cost Arsenal the Premier League title. The club cannot afford a repeat with a first league crown in 22 years still possible.

Should Gabriel miss time, Cristhian Mosquera is the likely deputy. Saliba—rested against Southampton—was pressed into the final 20 minutes anyway, underlining how thin the centre-back cover has become.

Arsenal’s European path is forgiving: Sporting are the weakest side left, and Bayern Munich, Paris Saint-Germain and Real Madrid are bracketed on the other half of the draw. Yet Arteta refused to lean on absentees as an excuse for recent cup failures. “Let’s look at ourselves in the mirror,” he said. “We go to Portugal with clarity.”

The pain of two cup exits in a month lingers, but the manager insists the remaining twin objectives—Premier League and Champions League—remain historic and attainable. First, he must wait on a thigh scan.