Atletico Madrid midfielder Saul Niguez reportedly had trouble breathing after a heavy challenge from Bayer Leverkusen's Kyriakos Papadopoulos during Tuesday night's Champions League clash.
Saul had to leave the pitch after the incident, which happened in the first half, and remained in Germany for treatment when the rest of the squad flew back to Madrid after the 1-0 defeat.
A statement published on the club's official website said: "Saul Niguez, under observation at the Leverkusen Clinic, is in stable condition.
"There has been an improvement in the kidney trauma with internal bleeding which he suffered, having received a bang in the ribs.
"The youngster will remain at the clinic for observation, depending on how he recovers from the physical pain."
AS reported that the midfielder Greece international defender Papadopoulos caught Saul in the stomach with a knee after 21 minutes.
The Atletico player received treatment and returned to the pitch before being substituted, suffering breathing problems, 20 minutes later.
He later wrote on Twitter: "Thanks to everyone for the messages to cheer me up. I will return even stronger."
Muchas gracias por todos los mensajes de ánimo, volveré con mas fuerza ! Vamos @Atleti ��⚪️�� pic.twitter.com/YEb6B49nhO
- Saúl ñiguez esclapez (@saulniguez) February 26, 2015
Atletico's coaching staff were unhappy with Leverkusen's physical approach, with Diego Simeone and assistant German Burgos voicing their anger to home coach Roger Schmidt.
Leverkusen committed 14 fouls in the first half and Atletico three, but Czech referee Pavel Kralovec issued one yellow card to each side during that period.
After the break, the official showed a second yellow card to midfielder Tiago and booked centre-back Diego Godin, meaning both players will miss the second leg next month.
In better news for the club, Koke is close to a return from the hamstring problem suffered in the 4-0 derbi win over Real Madrid earlier this month.
The club's official website reported that he could be back for Sunday's game at fifth placed Sevilla.
"The home-grown Rojiblanco youngster, in another step in his recovery, trained with complete normality at the Ciudad Deportiva de Majadahonda," a statement said.