Last season's beaten semi-finalists got off to a winning start in the Basque country as they shaded a tight game at the San Mames stadium in the first of eight group matches.

Mikel Arteta's side, chasing a first major trophy since 2020 and their first ever Champions League win, ground out their victory with a typically sturdy defensive display and their late goals.

Martinelli scored in the 72nd minute, 36 seconds after coming off the bench, before Trossard added a second after 87 minutes, each setting the other up.

Athletic, back in the competition for the first time in 11 years, were on top in the early stages, buoyed by boisterous home support.

Their coach Ernesto Valverde lined up with an entire team of players making their Champions League debut, including talisman striker Inaki Williams but not his injured brother Nico Williams.

Arsenal started with four Spaniards, including two Basque midfielders in Mikel Merino and Martin Zubimendi, who used to play for Athletic's rivals Real Sociedad.

The Gunners, missing several regulars including Bukayo Saka, began to turn the tide through the movement of Viktor Gyokeres and Noni Madueke's persistence on the right wing.

Swedish striker Gyokeres had an effort saved by Unai Simon and then headed narrowly wide of the post after reaching a dangerous cross by Jurrien Timber.

Alejandro Berenguer pulled a shot wide of the far post with Athletic's first real chance shortly before the break.

Gyokeres, sporting a bandage after a clash of heads left him bleeding, nodded Declan Rice's free-kick wide when well placed.

Williams was denied by David Raya and Merino headed straight at Simon at the other end as the match stepped up a gear.

Arteta brought on Martinelli for the ineffective Eberechi Eze and immediately found the goal he craved as the Gunners countered at pace.

Martinelli ran on to Trossard's pass, took a fine touch to leave Andoni Gorosabel trailing in his wake and then fired under Simon.

The Brazilian was involved again in the second, cutting the ball back for Trossard, who beat Simon with the help of a deflection.

Arsenal reinforced heavily in the summer to increase their squad depth, particularly in attack, and it paid dividends here.

Arteta brought on two more new arrivals in Christian Norgaard and Piero Hincapie for their debuts late on.