Neymar will miss Tuesday's trip to Old Trafford with a long-term foot problem while the Parisians were dealt a further blow after Cavani sustained a hip injury in their 1-0 win over Bordeaux on Saturday.
That leaves Mbappe as the Ligue 1 side's most high profile forward for the Champions League last 16
Speaking to reporters ahead of the game on Tuesday, Tuchel said: "You can't place all of the burden and pressure on him. It's not his job to replace Cavani and Neymar and he can't play all three roles at once. I want us to provide him
"You cannot replace quality players, and we cannot replace Neymar. Manchester United could not replace [Paul] Pogba or [Marcus] Rashford. We cannot replace Neymar."
Former manager Jose Mourinho led United through the group stages of the competition, but since his
They were sixth place in the Premier League and eight points off of Arsenal in
"Manchester United are completely different," Tuchel added. "They get a lot of early leads. To play at this moment against this Manchester United is one of the toughest challenges we could have. Since the draw, things have completely changed."
United are unbeaten in all competitions under Solsjkaer, the Norwegian winning 10 and drawing one of his 11 games in charge — the only slight blemish on his record coming in a 2-2 home draw against Burnley.
Solskjaer has received particular praise for rejuvenating the form of striker Marcus Rashford who has scored six goals in 10 games since the caretaker boss was appointed, despite having a marginal role under Mourinho.