Arsenal manager Arsene Wenger has admitted that he has been surprised by Joel Campbell's form since breaking into the first team.

The Costa Rica international has been on Arsenal's books since 2011 but spent the first years of his Gunners career out on loan before finally getting his chance this season.

Injuries to the likes of Danny Welbeck, Alex Oxlade-Chamberlain and Theo Walcott opened the door for Campbell, and Wenger revealed that his performance in the Champions League victory over Olympiacos in December confirmed his place as a regular first-team player.

"Yes [I have been surprised by Campbell]. You want to be surprised by the quality of your players and where he surprises you is in the intensity of his teamwork. I have the choice on the flank between many players and he is one of the players who has come in at a very sensitive period," Wenger told reporters.

"I flat-refused to let him go because I saw something in him that gives you belief, because of his attitude. It is true he had players in front of him and you think 'can you block a player like that?' But he was four times out on loan. He came back and you think 'do I let him go out on loan again?'

"I thought always the attitude was there. But after Olympiacos, in a game where we had a knife on the throat, he turned up with something special - great teamwork and a pass for [Olivier] Giroud on the second goal that was massive for us. And then I thought 'he can play under pressure as well, which is important for us'.

"What is very interesting in Campbell's case is that he looks like he has gained more confidence and authority in his game and in the final third he is a dangerous player and he has shown that. When you look at the player and the number of goals he scores and the number of assists, on that front you must say he has been very positive and he works hard for the team."

Campbell is in line to start for Arsenal when they take on Liverpool at Anfield on Wednesday evening.