You are confusing facts with memories. 4 wins without conceding a goal turned into losing 2 of our next 4 after signing Sanchez. Scoring in 14 out of 15 games turned into drawing a blank in 3 out of 6 after signing Sanchez. Those are facts.
The Christmas games are irrelevant to my point. I said our attack got weaker because we shoehorned Sanchez in place of Martial. Against Leicester Martial started and we were winning, Mourinho subbed him for Rashford (who was shockingly bad) and we ended up drawing. He then missed the game against Burnley and was only on the bench against Southampton. He came back into the team against Everton, scored, and we won. He started against Stoke, scored again, and got an assist, and we won. Then he started against Burnley, scored for the third game running and we won again. He was easily our best attacker at that time and winning us games week after week.
Mourinho then sign Sanchez and insists on him starting every game in Martial's position. We promptly put in one of our worst performances of the season against Tottenham and in our next away game lost to Newcastle. We went from losing 2 of our previous 15 league games to losing 2 of 3 after signing him. From then until the end of the season we scored less goals and got worse results than we did before we signed him. Naming the occasional good performance from him in four months is meaningless. Virtually every premier league footballer, even the most limited, will put in a good performance every couple of months. It's the exact kind of ridiculously low bar that Rooney defenders used to use when confronted with evidence of his performances and the effect they were having on United. It says a lot that people are already resorting to this with regard to Sanchez.