Buffon, 40, and compatriot Marco Verratti are both banned for the Anfield clash with last season's beaten finalists because of suspensions incurred last campaign, while Dani Alves and Layvin Kurzawa are both out injured - and US global Timothy Weah is not included.
Tuchel's six straight competitive wins since taking charge broke Carlo Ancelotti's club record from 2011 , with Julian Draxler taking his chance to impress in the absence of rested Neymar and suspended Mbappe.
"However, I say that whether we play in Nimes, or Guingamp, or at home to Saint-Etienne, every game is hard". Tuchel told the club's official website: "I'm very happy with this victory, our sixth in a row since the start of the season".
Cavani converted a second-half penalty he won himself with Di Maria adding gloss to the scoreline with a thumping strike before teenager Diaby opened his Ligue 1 account in the 84th minute.
And Tuchel was impressed as he watched his side secure their fifth triumph in five league matches without his star duo.
And Liverpool boss Jurgen Klopp will put out his strongest side for today's Premier League game against title rivals Tottenham at Wembley (12.30pm). "Together, we decided that he would not play", Tuchel said.
Adrien Rabiot battled to win the ball in the centre circle and Marco Verratti's lofted pass into the box was nodded into the air by Draxler, who improvised to head a fine finish over Stephane Ruffier.
Without Kylian Mbappe and Neymar, PSG were still too hard to restrain for Saint-Etienne.
"We know that Anfield is one of the toughest trips in Europe".
The offside flag ruled out Timothee Kolodziejczak's own goal but PSG added a 76th-minute third when Di Maria lashed in from close range, substitute Diaby then making the most of Cavani's mishit to wrap up the scoring.
Other two Champions League contenders will feature in Saturday's action on road, with Lyon visiting Caen and Monaco facing Toulouse.