The former Real Madrid man struggled to stay fit during his debut season at the Parc des Princes but has blossomed into a key player for Christophe Galtier this season.
While coming under fire for some inconsistent performances, Ramos has scored two goals and provided one assist in 36 appearances, starting 22 Ligue 1 games and playing every minute of Les Parisiens' run to the last 16 of the Champions League.
The 36-year-old also completed the full 90 minutes in last weekend's 2-1 win over Brest, and he would have been expected to hold his place in the backline for the visit of Rennes on Sunday afternoon.
However, PSG have announced in a medical update that the centre-back was not able to train on Friday due to his calf issue, which he supposedly sustained during a game against the club's academy prospects.
Ramos has not been ruled out of the clash with Rennes just yet, but the former Spain international's concern represents yet another untimely defensive injury for Galtier to factor in.
Nordi Mukiele and Achraf Hakimi are both training individually as they recover from thigh injuries, while Marquinhos is struggling with a rib problem sustained against Bayern Munich in the Champions League.
Presnel Kimpembe, meanwhile, is out for the rest of the season due to a torn Achilles tendon, but Galtier refused to rule Ramos or Marquinhos out of Sunday's game in his pre-match press conference.
"Sergio felt some discomfort and we wanted to stop his session to avoid taking any risks. He will be with us tomorrow for the last session, we will review the situation then, as we did with Marquinhos," Galtier told reporters ahead of a difficult selection dilemma.
Should Ramos and Marquinhos both miss out, 17-year-old El Chadaille Bitshiabu looks set to partner makeshift centre-back Danilo Pereira in front of goalkeeper Gianluigi Donnarumma.
Mukiele and Hakimi's absences will likely see Timothee Pembele start on the right-hand side of the backline, and PSG's injury crisis also extends to the final third of the field.
Already known to be without Neymar as he recovers from ankle surgery, PSG also revealed that Carlos Soler was restricted to indoor work on Friday due to muscular discomfort.
The Spaniard opened the scoring in last week's win over Brest, where Franck Honorat equalised before Kylian Mbappe scored the winner with just one minute left on the clock.
Either Vitinha or Renato Sanches could come in for Soler if the former Valencia man is ruled out, but Galtier can also introduce Hugo Ekitike into the attack alongside Mbappe and Lionel Messi.
PSG hold a 10-point lead over Marseille at the top of the Ligue 1 table and can temporarily extend that lead to 13 with a win over Rennes before Les Olympiens face Reims later on Sunday.