January signing Krystian Bielik made his first start in Arsenal colours on Tuesday night , but the teenage prodigy could do nothing to stop Derby County beating the Gunners at the iPro Stadium . Bielik has been told that he will not play for Arsenal 's first team this season as Arsene Wenger looks to bed the 17-year-old into the squad slowly , but he has been used by the youth sides , although not as an Under 21 starter until Tuesday . The Poland midfielder put in quiet but tidy performance before Derby 's Ivan Calero smashed in a corner to steal the show and all three points in the Barclays Under 21 Premier League . Krystian Bielik breaks away from Alban Bunjaku of Derby County on his Arsenal youth team full debut . Bielik signed for Arsenal for £2million in January but has been told he is not yet ready for the first team . Bielik , signed from Legia Warsaw in January for £2million , even saw a header cleared off the line in the closing stages of the game but a dream youth team debut was n't to be . The defeat for Arsenal meant Derby leapfrog them in the U21 Premier League on goal difference , although the Gunners have three games in hand . Derby were on top from the start and went close through Shaquille McDonald , Daniel Crowley and Callum Guy before Jamie Hanson curled two free-kicks against the crossbar . George Dobson aims a pass away from the challenge of Alefe Santos of Derby at the iPro Stadium . Daniel Crowley tries to escape the attentions of Bunjaku , but Arsenal lost and were leapfrogged by Derby . Arsenal looked like holding out but the deadlock was finally broken by Calero 19 minutes from time . Bielik had his effort from a Daniel Crowley corner cleared off the line with a minute to go but the evening was Derby 's .
Ivan Calero scored the only goal as Arsenal were beaten by Derby County . this season signing Krystian Bielik made his U21 debut for the Gunners . The £2million teenager saw a headed effort cleared off the line late on .