Gabriel Martinelli scored just 36 seconds after coming on as a substitute to fire Arsenal to an impressive 2-0 win over Athletic Club in their Champions League opener in Bilbao.

Martinelli had just been introduced as a replacement for Eberechi Eze when he raced on to fellow sub Leandro Trossard's ball over the top.

The Brazilian took one touch with his head, and then two further touches at speed, before firing under the body of goalkeeper Unai Simón with 18 minutes remaining.

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Then with three minutes to go, Martinelli turned provider when he pulled the ball back for Trossard whose shot at goal flicked off the upright and into the net.

It has been 132 days since Arsenal were eliminated by Paris Saint-German in the semi-finals, and Mikel Arteta's players began their bid to win the Champions League for the first time at a hostile San Mames Stadium.

With Arsenal armed with eight new players for this season's competition -- with five of them starting here -- Atletico Club manager Ernesto Valverde said on the eve of Tuesday's fixture that the Gunners must be considered as one of the favourites to win the title.

The home side, themselves back in the big time after a decade away, enjoyed an encouraging opening before Noni Madueke, occupying the right side in the continued absence of Bukayo Saka, started to cause them bother.

Eze looked set to capitalise on Madueke's cutback midway through the first half only for a last-ditch sliding tackle from Andoni Gorosabel to deny the former Crystal Palace man.

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Moments later, Viktor Gyökeres called Simon into action for the first time with a low but comfortable shot for the Athletic goalkeeper to deal with. The Sweden international might have forced another save from Simon moments later only to see his diving header from Jurriën Timber's wicked cross fizz wide.

Madueke's direct play then won a free kick, which was taken by Declan Rice and headed over by Gabriel. But the stand-in captain, with Martin Odegaard sidelined through injury, took out teammate Gyokeres in the process.

Gyokeres required treatment and a head bandage, and the enforced break took the sting out of Arsenal's momentum, with Bilbao ending the opening half in the driving seat albeit without calling David Raya into action.

Seven minutes into the second half and good hold-up play from Gyokeres allowed Madueke the chance to race round Adama Boiro, but Simon was equal to the England man's left-footed stab at goal. A handful of minutes later, Madueke cut back inside the area on his left foot but his low drive was comfortably saved by Simon.

The game was starting to become more open and Iñaki Williams registered his side's first shot on target shortly before the hour mark. Five minutes later and Mikel Merino headed straight at Simon from Eze's cross.

Gyokeres made way for Trossard with Eze following suit -- and the two changes linked up to propel Arsenal ahead.

Martinelli, who dropped to the bench for the win against Nottingham Forest and again here with Eze preferred ahead of him, provided a stark reminder of his worth with a fine breakaway goal.

And then Martinelli was too hot to handle for Gorosabel when he beat the full-back for pace before picking out Trossard who converted with aplomb to put the gloss on a fine European evening for Arsenal ahead of their Premier League clash against Manchester City at the Emirates on Sunday.

Information from PA was used in this report.