The Brazil international sat out Saturday's session because of gastroenteritis and his condition will be assessed later, a PSG statement read.
Teenage defender Timothee Pembele will be unavailable after returning a positive coronavirus test, with the 18-year-old now self-isolating.
The champion will also be without Keylor Navas and Ander Herrera for Sunday's Classique, with neither player fully fit.
Colin Dagba (thigh) and Juan Bernat (knee) are still out, while Abdou Diallo remains sidelined because of coronavirus protocols after testing positive a week ago.
PSG, which is three points behind league leader Lille after 23 games, is still the favourite for Monday's (AEDT) game against struggling Marseille.
Marseille suspended coach Andre Villas-Boas this week after he offered his resignation in protest against the club's "sporting direction", with Olivier Ntcham signed from Celtic without his consent.
Marseille, which is ninth, has won just once in their previous 10 matches in all competitions and lost 2-1 to PSG in the Trophee des Champions match on 14 January (AEDT).
This will be the 100th competitive meeting between the teams, with PSG leading the head-to-head record with 44 wins to 33, 18 of which have come since Qatar Sports Investment took over the club in 2011.
However, Marseille won the reverse game this season 1-0 in September and the potential loss of Neymar could be key, the 29-year-old having scored 11 goals in his previous nine games in all competitions.