• Signing a right winger shouldn't influence other needs across the pitch in a significant way, methinks. A behemoth like United can and should be able to sign 3-ish good players in a window given the resources and scouting/recruitment personnel at the club's disposal, and depending on how you view the position, a defensive midfield signing falls within that range.
• The need for a defensive midfielder depends on our first choice combination, how they interact with each other, the overall midfield scheme and so forth. If we choose to use Pogba and Fernandes as the two most advanced midfielders, that position (triangle between them as well as the centerbacks) becomes a priority because Matić usually performs best when he is frequently rested (and is no spring chicken at the stage of his career), and Fred/Tominay aren't genuine defensive midfielders in terms of passing tendencies and positional application. Even if Pogba leaves and we employ a double pivot setup with Bruno as the #10, someone like Partey or Saúl could take our midfield up a notch as medium-long term partners for Fred/McTominay. Sometimes a player can provide greater balance and make a profound impact, even in a good team — like Martínez at Bayern under Heynckes (who already had the likes of Gustavo, Tymoshchuk, Kroos, Schweinsteiger in midfield), so one must keep an eye out for improvement.
• Luckily for United, a handful of good (or potentially really good) defensive/holding midfielders could be available over the summer — including youngsters with immense potential/instincts, like Luís from Benfica (linked with Milan and Nice in the winter window) or Camavinga from Rennes — who could be carefully introduced to the rough and tumble of hgih-stakes English football behind Matić over the next year or so, instead of being thrown into the deep end. Some of them might develop into world-class players who make a genuine difference over the long haul, so we would be wise to act proactively if the opportunity presents itself, IMO.
Mind, this is just how some of us view things, so there's bound to be a difference of opinion with others — also...maybe that assessment is flawed and we don't necessarily need a defensive midfielder, maybe Fred/McTominay/Matić will suffice in the configuration Ole employs, maybe Fred/McTominay grow into more disciplined defensive roles, maybe Garner steps us under Carrick's tutelage...who knows, really?