Atletico Madrid have confirmed that full-back Juanfran has sustained a grade II muscle injury to his left thigh.
The 32-year-old was forced off in the 55th minute of Tuesday's 1-1 Champions League quarter-final second-leg draw with Leicester City at the King Power Stadium.
Juanfran was sent for scans that have revealed damage to his quadriceps, with Atletico confirming the news via their official website.
"Our player will undergo physiotherapy and rehabilitation treatment," the club added.
The news is a big blow to coach Diego Simeone, who saw left-back Filipe Luis sustain a hand injury that forced him to field four centre-backs in the closing stages of the match.
The loss of Sime Vrsaljko to a knee ligament tear in March means Atletico could be facing a full-back crisis for the semi-finals.
Atletico face LaLiga matches against Espanyol, Villarreal and Las Palmas before the first leg of the Champions League last four early next month.