Danny Welbeck was taken straight to hospital after suffering an ankle injury during Arsenal's goalless Europa League draw with Sporting CP, head coach Unai Emery has confirmed.
Welbeck returned to the starting line-up for the match at the Emirates Stadium and wasted an early chance to break the deadlock.
His second opportunity saw the England international head straight at Renan Ribeiro, but the striker landed badly on his ankle and left the field on a stretcher.
Emery confirmed the injury was "serious" but the Gunners will wait to learn the full extent of the 27-year-old's issue.
"The worst news tonight is these injuries," the Spaniard told a media conference. "We think [they are] serious injuries.
"He has been taken to hospital. We are going to wait [for updates].
"Danny was working well. He is a very good person, his commitment to us is very big.
"This injury is very big for him, for us and for everyone."
Arsenal also saw defender Stephan Lichtsteiner depart with an injury with 16 minutes remaining, and Emery hopes that is not as serious as Welbeck's problem.
"Lichtsteiner is muscular," he added. "We will know [more] about his injury tomorrow."
|Solskjaer: Lukaku is big part of Man United squad|
|Napoli slam FIGC decision to reject Koulibaly appeal|
|We´re the hunters! - Kovac´s Bayern ready to chase down Dortmund|
|Chiefs safety Reid expected to play against Patriots|
|Chelsea not ready to rest Jorginho - Sarri|
|NFL receiver Fitzgerald has hole-in-one in round of golf with former president Obama|
|I´m really loving playing this game - Eagles QB Foles not ready to retire|
|Saints show no mercy, score 111 in rout of Timisoara Saracens|
|CAF Champions League Review: Khenissi double gets ES Tunis up and running|
|Edinburgh clinch home quarter-final, Glasgow also advance to last eight|
|Hazlewood to miss Sri Lanka Tests with back injury|
|Norwich City 3 Birmingham City 1: Canaries end winless run to go second|
|Hoffenheim 1 Bayern Munich 3: Goretzka goals close gap on Bundesliga´s return|
|Cowboys part ways with offensive coordinator Linehan|
|Payton´s message to Saints fans: Hurry up and make noise|
|Transfer talk no distraction for Inter, says Spalletti|
|Memories of 2015 still linger for Vunipola as England begin World Cup year|
|De Bruin´s 85th-minute try consigns Kings to eighth straight loss|
|Bielsa´s Derby County dossier detail ´pretty standard´ – OptaPro chief|
|Injury-hit Madrid lose Navas to adductor issue|
|Patriots coach Belichick still gets nervous ´every week´|
|Copa del Rey draw: Barcelona to meet Sevilla in quarter-finals|
|Kohli: No one more committed to India than Dhoni|
|Brook questions Khan credibility following Crawford agreement|
|Sad Pochettino admits Bielsa is ´wrong´|
|Simeone eyes LaLiga gain after Atletico´s Copa del Rey pain|
|Rossi a no-go for Tottenham as Pochettino rules out panic buys|
|Guardiola identifies ´three or four´ midfield targets|
|Nadal encourages patience in young stars after humbling De Minaur|
|NFL playoffs 2019: 10 Opta facts for Patriots v Chiefs|
|Robertson helping Klopp learn Scottish for Dalglish chats|
|Levante appeal over alleged ineligible Barcelona player rejected|
|NFL Playoffs 2019: 10 Opta facts for Rams v Saints|
|Lowry to take three-shot Abu Dhabi lead into final round|
|Training is not for anyone else - Klopp condemns Bielsa over ´spygate´ row|
|In other countries, everybody does it - Guardiola respect remains for spying Bielsa|
|Cilic battles back to join Nadal, Federer in fourth round|
|Klopp: Trent unbelievable for playing through pain|
|Faulkner provides super start and fabulous finish to sink Scorchers|
|Ref admits error in key foul call on Lonzo Ball in Lakers-Thunder game|
|Marina Granovskaia is working very hard – Sarri ´confident´ of January signings|
|Tuchel confirms Rabiot out of PSG contention|
|Madrid derby kick-off confirmed for February 9|
|Benzema fit to face Sevilla despite broken finger, Solari confirms|
|Bournemouth boss Howe says no offers received for Wilson|
|Kerber cruises after Wozniacki, Sabalenka upsets|
|Mbappe a great player and great person, says beaming Beckham|
|Gracia confident someone will step up if Doucoure leaves for PSG|
|Albrighton signs new Leicester deal|
|Niasse joins Cardiff on loan from Everton|
|Hendrick credited with Fulham strike|
|Western Sydney Wanderers 1 Adelaide United 2: Goodwin compounds Wanderers woes|
|Chahal and Dhoni carry India to ODI triumph|
|Levante report Barcelona for alleged use of ineligible player in Copa del Rey|
|Solskjaer regrets Sanchez´s Reading setback|
|Tomic brands Hewitt a ´liar´ as feud continues|
|Ruthless Nadal thrashes teenager De Minaur|
|Wozniacki enjoyed status as defending champion in Melbourne|
|Solskjaer backs Mourinho to return to football´s big time|
|Murray to miss Open 13 Marseille|
|Solskjaer´s United turnaround no easy task, says Hughton|
|The big Friday Premier League quiz|
|How James Harden´s 2018-19 numbers compare to his MVP campaign|
|It´s insane! - Tsitsipas excited to face ´legend´ Federer|
|Sharapova ends Wozniacki title defence|
|American teen Anisimova sends records tumbling at Aus Open|
|Lakers survive Ball´s gaffe to beat Thunder in OT|
|Federer expects ´athletic´ and ´attacking´ Tsitsipas clash|
|Alcacer is fit and available – Favre|
|Fantastic Federer breezes past Fritz|