Wouldn't rank him above Mario Gotze just yet to be honest (purely post Messi era, otherwise you'd have to include the likes of Cesc aswell). Easy to forget now, but there was a time when Gotze was being talked of as arguably the biggest German talent since Matthaus. Played a big role in 2 consecutive Bundesliga titles, best player in the 2009 U17 European Championship, Bundesliga team of the season, 'Golden Boy', performed really well vs Brazil in a highly scrutinized match - Gotze vs Neymar for the best teenager in football, and so forth. Martial's body of work at United is impressive, but too condensed to be making such proclamations. Then again, it's Redknapp, and the Sky hype machine. Doubt he's watched Gotze at 18 or 19.
Although, like Gotze, Martial seems almost chillingly mature and intelligent for his age, like a frost wolf, something that
@bosnian_red mentioned previously. Could be a treat to watch over the years.