The France and Uruguay internationals both suffered injuries and were substituted during the 4-0 home win over Toulouse in Ligue 1 on Sunday.
The French champions released a statement and said Mbappe will miss around four weeks of action with a hamstring problem, while the club are hopeful Cavani will recover from a hip issue within three weeks.
The Ligue 1 giants added that defender Abdou Diallo had suffered an eye injury, but it is not thought to be serious.
However, the injuries leave PSG short up front to face Metz in the league before the international break and potentially the start of the Champions League group stage as well, with the draw set for Thursday.
Neymar is suspended for the first three fixtures in Europe this season for insulting match officials on social media after PSG were eliminated by Manchester United in the competition last season.
Before the match, PSG coach Thomas Tuchel said Neymar would not feature until his future is resolved and sources have told ESPN FC reintegration talks have already taken place between the club and the Brazilian.
With time left in the transfer window, sporting director Leonardo could look to reinforce the attack with a signing, or turn to the likes of forgotten man Jese Rodriguez to provide extra depth to Choupo-Moting.