@Andy @ScouseSte Seems a bit strange to place such an emphasis on one game where Neymar was missing and suggest that his "failure to step up" proves he's not up to it.
Much better to look at last season when Neymar was out of the team for a prolonged period and see what happened to Mbappe then. Between Jan 24 and the end of the season, in games Neymar missed, Mbappe's PB average was very similar to his seasons average overall (108 Vs 110 across the season), while he scored 19 of his 37 goals for the season in this period (in 17 games, with the other 18 coming in the 20 games prior to Jan 24). He also claimed 5 of his 9 assists in these 17 games, with the other 4 coming in the remaining 20 games he played.
In summary, his goals and assists contribution increased slightly without Neymar being in the team (1.1 goals per game Vs 0.9, 0.29 assists per game Vs 0.2). His PB average wasn't affected, although he scored 100+ slightly more frequently (in 65% of games Vs 60%). It is also worth considering that playing in the same team as Neymar, with the creativity he has, may lead to increased PB average scores for a player like Mbappe.
In summary I would say that Mbappe more or less continued to do his thing in the absence of Neymar. His numbers improved slightly but this sample size still isn't big enough to make concrete conclusions. What is clear though is that a player who was 20 at the time was shouldering a huge responsibility for one of the world's most high profile teams and he certainly wasn't shrinking into the background.
Hope that information helps traders to make their own minds up - even though it's not necessarily conclusive, I feel better having looked into it rather than taking one game as evidence as to whether or not he can "step up".