It doesn't help that they're doing everything they can to tank Mantha's value. Zero powerplay time, spent much of the first 25ish games on the 3rd line, and now recently bumped to the 4th line. His boxcars are fine for a Top 6 forward with no powerplay time, and his metrics are among the better on the team across the board.
I'm not saying this to wash over his slump periods (one of which he is certainly in now). He's a player with very pronounced peaks and valleys over the course of a season, and when he struggles he definitely struggles. But the Capitals simply aren't doing themselves many favors when it comes to getting the most value out of Mantha, and it's not likely to become easier to do so with Wilson back in the line-up.
It is worth remembering that Mantha is 3rd on the team in 5v5 goals (8) behind only Ovechkin (12) and Dowd (10) of all people. He's also tied-3rd on the team in 5v5 points (18) behind Ovechkin (23), Sheary (19), and tied with Kuzy (18).