Rooney was the reason LVG got the sack.
Van Gaal sold RVP and Chicharito, chose not to sign Falcao on a permanent and then proceeded with Rooney as the sole striker from August-January until Rashford's emergence. During that time, Rooney had THE worst period of his career performance wise. He was absolutely rubbish. Couldn't score a goal to save his life and LVG still wouldn't drop him to play somebody like Martial or Depay down the middle. His performances as a whole were uninspiring.
It was only when Rashford came on the scene did things start picking up again. We went onto win the FA Cup thanks to that.
But ultimately the reason we failed to achieve top 4 was because of that August-January period where Rooney went something like 3 months without a goal in the league. A truly abysmal record for a striker. If Van Gaal didn't sell Hernandez for missing a penalty, didn't allow Di Maria to join Paris and didn't sell Van Persie for being "too old" (despite him then signing Schweinsteiger a few weeks later), we would have probably won the title over Leicester.