The 21-year-old centre-back spent last season on loan at Charlton Athletic, earning promotion to the Championship in May by winning the League One play-off final at Wembley.
Krystian joined us from Legia Warsaw in January 2015 and made his senior debut in the League Cup in October 2015 against Sheffield Wednesday.
The Polish international was part of the Arsenal side who were victorious in the Under-21 Premier League Division 2 play-off final during the 2015/16 season.
Since January 2017, Krystian has spent time on loan at Birmingham City and Walsall, before his spell at Charlton on loan last season where he was primarily deployed in a holding midfield role.