Borussia Dortmund have stated teenage star Konstantinos Karetsas is ‘doing well under the circumstances’ after being taken to hospital with circulatory problems.Karetsas, 18, had to be substituted 27 minutes into their Champions League tie against Villarreal on Tuesday night.The Greek attacker was seen kicking the ball back to his team-mate before sitting down on the pitch in discomfort.Dortmund medical staff entered the pitch to attend to Karetsas, who could then be seen lying on the pitch.Karetsas was ultimately stretchered off the field of play, with Dortmund later confirming he was suffering from ‘circulation problems’.’Konstantinos Karetsas had to leave the pitch in the game against Villarreal due to circulation problems,’ a club statement read. Konstantinos Karetsas slumped to the pitch during Dortmund’s Champions League tie with Villarreal The teenager was taken to hospital with Dortmund stating he was suffering from ‘circulation problems”Kosta is doing as well as can be expected given the circumstances, and he is on his way to the hospital for further examinations.’ German newspaper Bild reported that Karetsas had missed training on Monday due to fatigue.Karetsas joined Dortmund last month in a £27million move from Belgian side Genk in the summer.The teenager’s fellow Greek Giannis Konstantelias had joined for a similar fee earlier in the window, but suffered an ACL injury on his debut in a major blow to Dortmund.That injury led to Dortmund loaning in Ethan Nwaneri from Arsenal.Nwaneri came off the bench in place of Karetsas on Tuesday as Dortmund began their Champions League campaign with a victory.Serhou Guirassy scored twice and also put through his own net in a 3-2 win by the Bundesliga side.A Renato Veiga own goal had put Dortmund ahead before Santiago Mourino levelled for Villarreal.Guirassy netted twice in the closing ten minutes to earn a victory, before an injury time own goal.