Graeme Souness has warned Liverpool about the threat of Gabriel Martinelli as Jurgen Klopp’s side prepare to meet Premier League leaders Arsenal.Arsenal news, exclusives and analysisLiverpool face the unenvious task of taking on Arsenal in their own back yard on Sunday as the Emirates Stadium plays host to a mouth-watering Premier League clash.Arsenal have won nine of their first 10 games this season to climb to the top of the English top-flight, one point ahead of defending champions Manchester City.Mikel Arteta’s side will look to make it five wins in a row this weekend, having dropped just three points all season, losing at Manchester United last month.Liverpool, meanwhile, sit just ninth in the Premier League having amassed 10 points from their first seven league games.Souness believes Liverpool are facing their ‘biggest test’ since Klopp’s appointment in 2015 and admits things won’t get much easier as they prepare for a clash with in-form Arsenal.The legendary Scottish midfielder says his former club will need to pay particular attention to Brazilian winger Martinelli, who he says is destined to be a ‘real player’.To view this video please enable JavaScript, and consider upgrading to a web browser that supports HTML5 videoMartinelli has been in brilliant form for the Gunners and will relish the chance to expose Trent Alexander-Arnold’s well-documented defensive deficiencies.Writing in a column for the Daily Mail, Souness said: ‘We will see in the next eight days, with the game at Arsenal on Sunday and Manchester City at Anfield next Sunday, how Liverpool’s players deal with the biggest test they have known as a group.‘Gabriel Martinelli will be a big part of the threat to them at the Emirates.