Atletico Madrid have confirmed Marcos Llorente aggravated a thigh injury in Tuesday's Champions League defeat to Liverpool and is being assessed.
The 26-year-old initially sustained the injury during Atletico's 2-0 win over Barcelona on October 2 and was forced to pull out of Spain's squad for their recent World Cup qualifiers.
He was not fit enough to start the dramatic 3-2 loss at the hands of Liverpool but was introduced from the substitutes' bench for the final 10 minutes.
Llorente pulled up late on in the game and is now facing another possible spell on the sidelines.
A statement on Atleti's official website on Wednesday read: "Marcos Llorente has suffered a relapse in the muscle injury he suffered in his right thigh.
"The player has undergone tests at the University of Navarra Clinic, in which it has been detected that the scar from his previous injury had partially opened.
"At the end of Tuesday's game against Liverpool, the player expressed discomfort to the club's medical services in the same area where he had suffered his injury.
"Today he will begin medical and physiotherapy treatment and will soon resume the rehabilitation process. His return to competition is pending evolution."
Marcos Llorente suffers relapse of right thigh muscle injury.— Atlético de Madrid (@atletienglish) October 20, 2021
The versatile Llorente has featured in all 11 of Atletico's matches this season, starting each of those prior to the European clash with Liverpool on Tuesday.
Only goalkeeper Jan Oblak (990) has played more minutes for the LaLiga champions in all competitions this term than Llorente's 886.
While he has yet to score or assist a goal, the 12 chances created by Llorente is bettered only by Rodrigo de Paul (13) among Atleti players.
Diego Simeone's men host surprise leaders Real Sociedad on their return to LaLiga action on Sunday following a three-week break from domestic action, before travelling to Levante four days later.