Arsenal substitutes Gabriel Martinelli and Leandro Trossard pounced in the final stages to secure a 2-0 victory at Athletic Bilbao in their Champions League opener on Tuesday.

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.

"We started to grow throughout the game. In the second half we were more fluent, dominant, and the finishers made the impact for us to win the game," Arteta told reporters.

"(The substitutes) are at least equally important, if not more important.

"We have discussed that -- the finishers are going to be more important this season sometimes than the starters, and we can change the game then, when teams start to drop off."

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.

- 'Fully deserves it' -

Arteta, who hails from the Basque country, was praised as one of the best coaches in the world by his counterpart Valverde and proved it with substitutions which changed the game decisively to ensure a happy return to home soil.

The Spaniard brought on Martinelli for the ineffective Eberechi Eze and immediately found the goal Arsenal 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.

"(Martinelli) fully deserves it," said Arteta. "I adore Gabi, his attitude, his commitment, his positivity and what he's willing to do for the team.

"We value all the qualities he has and what he brings to the team and I'm so happy that today he decided the game for us in two actions.

"These are the moments that are hopefully going to give him the confidence (he needs)."

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 with the points safe.

"The result doesn't reflect what the match was like, because I thought we had a good game," said Williams.

"We gave every drop of sweat and there are seven games left -- Athletic didn't come here to enjoy the competition but to compete in it."