Maybe he hasn't found a player that suits his taste, with the upcoming transfer market he can recruit players according to his taste and make his squad better than before, also besides that Amorim opened his mouth about the disappointment experienced by his fans if he understands it, but there is no instant way to make his squad better everything takes time as Amorim himself said.
https://www.manutd.com/en/news/detail/ruben-amorim-has-message-for-fans-ahead-of-wolves
https://www.manutd.com/en/news/detail/ruben-amorim-has-message-for-fans-ahead-of-wolves
I don't think United will be active in the January transfer window because it will be more costly to get the right players during this period. Also, most players prefer moving in the summer window, and the kind of players available in January window are fringed players with less game time at their clubs. Two areas that I think needs reinforcement are left wingback and a top striker. They've signed Diego Leon which will be available next season, so Amorim just has to make do with his crop of players till the season ends and bring whoever he wants in the summer window.