It has nothing to do with depth, Pens made the playoffs last season despite significant injuries. The problem is the players already there under-performing.
They lost Cullen (13 goals, 31 pts), Kunitz (9 goals, 29 pts), and Bonino (18 goals, 37 pts) from their bottom six.
To make up for it, they are now using the following players:
Tom Kuhnhackl (on pace for 10 pts) in a more regular role after being frequently scratched last season
Carter Rowney (also on pace for 10 pts)
Ryan Reaves (on pace for 8 pts, hasn't scored in 32 games)
Riley Sheahan (on pace for 29 pts, which isn't terrible when compared to the others but is still coming up short in goals when compared to Bonino)
I don't think they're underperforming. Underperforming would be indication that these players had a history of scoring and just weren't getting it done this year. Considering Kuhnhackl's, Rowney's, and Reaves' career high point totals are 16, 7 (in very limited action), and 12 respectively, I'd say it's more of a case of not having the offensive ability vs underperforming.