He can be a frustrating player, but I think Mourinho's handling of his potential is the most frustrating thing about his time at United. The signs indicate he's on the cusp of being one of the best strikers in this league. This season his goal record has been quite good - 16 goals with roughly 14-15 games left of the season (depending on how far we go in the cups) suggests he's likely to hit 21-24 for the season. Comparing him to other strikers for teams we're competing with he's clear of Firmino and on par with Kane and Abraham.
He's undoubtedly a better player technically than Abraham and has had to use that ability to drop very deep at times this season to compensate for our lack of creativity in midfield. He prefers playing one-twos and getting involved in quick exchanges with players to create opportunities and that's why he's always enjoyed playing with other technical players. Due to Pogba's injuries, Martial hasn't had the chance of playing as a striker with a proper provider in midfield, and the difference Bruno made to Martial's game is clear as day. He's scored 4 in the last 6 with Bruno alongside him and that's without Pogba and Rashford too.
As much as we deserve a consistent 25 goal season from Martial (which I'm expecting next season), he also deserves the chance to play upfront in a team that isn't disjointed and has other players on a similar wavelength to him before we can fully judge him. He's got a goal or assist every 120 mins this season and has the 2nd most goals against teams in the top 6 over the last two seasons after Aguero. I've bolded that because for me that's a key indicator as to whether a player truly has the tools of being world-class. Lukaku scored 1 goal for us against the big 6 in his 2 seasons here whereas Martial in fewer games has managed 8 goals in the league against them since the beginning of 18/19. He holds his own playing with or playing against top players, and his last game before the break against City was as complete a performance as you'd hope to see from a United striker against a team like that. He's good for the team, and he's better than a significant proportion of the players that have been mentioned as potential replacements for him.
Finally, I recently came across a tweet that we haven't had two players score 20 goals for us in a season since 2010/2011. Since then we've had Rooney, RVP, Falcao, Mata, Di Maria, Lukaku, Sanchez at the club. Rashford and Martial are going to break that duck together, and they deserve their respects for doing so in my opinion.