Neither Jackson or Watkins are good options, it's better for Manchester United to wait for the next transfer window in order to get better striker. The task of Manchester United in this season is to survive and not relegated to Championship.
Jackson is the worst possible option they can go with if they rush. I see that they want to get everything and be done, but if they end up with Jackson they would be horrible. I see that Cunha and Mbuemo was their only transfers so far, at least only two that I have seen mean anything, not sure anything else. But if we focus on the reality, that wasn't enough at all and they overpaid for the players.
We need to consider things changing a lot worse and they need to get more players. However, just because they "need" to, doesn't mean they should rush, because if they rush then they won't get any good return on their investment. Look at how Cunha was last game, he didn't even look like he worths that much money they paid for him.