manchester united beat their rivals city 4-2 at old trafford on sunday .
it led to supporters claiming manchester had been turned red once more .
united had lost their four previous derbies against their great rivals city .
louis van gaal appears to have turned united 's fortunes round this year .
sportsmail 's ian ladyman examines the issues after the fierce derby .
read : five things van gaal has done to transform results at man utd .