Neymar was absent from the PSG squad amid continued uncertainty over his future at the club and Thomas Tuchel lost striker Cavani 14 minutes into a goalless first half.
Eric Maxim Choupo-Moting came on for the Uruguayan and opened the scoring in the 50th minute with a superb solo strike, before defender Mathieu Goncalves turned in an own goal.
The result was completely overshadowed, though, as Mbappe appeared to pull his hamstring when chasing down a pass, with the France forward heading straight off the pitch before Choupo-Moting and Marquinhos added further goals.
PSG, beaten 2-1 at Rennes last week, quickly cranked into gear and Toulouse goalkeeper Baptiste Reynet made a pair of smart saves from Mbappe in the opening four minutes.
But its strong start faded fast and PSG was dealt a blow when Tuchel saw Cavani hobble off after an attempted volley, with Choupo-Moting introduced as his replacement.
Choupo-Moting was acrobatically denied by Reynet in the first minute after the restart but the keeper could do nothing about the Cameroon international's next effort.
After collecting the ball in a packed penalty box, Choupo-Moting superbly span into space and thumped a low drive in off the left post.
PSG's second goal came five minutes later, Pablo Sarabia's attempt deflecting in off Goncalves, who then gave away a 71st-minute penalty for handball after Mbappe pulled up on his 100th Ligue 1 appearance.
Reynet dived to his right to deny Angel Di Maria from the spot, but PSG added a third goal four minutes later as a sweeping move ended with Juan Bernat teeing up Choupo-Moting for a first-time finish.
Choupo-Moting was denied a hat-trick by Reynet before Marquinhos' header completed the scoring on his 250th appearance for the club, but PSG will be hoping for good news on Cavani and Mbappe.