Makelele and Naybet in summer 2000 instead of Veron and Blanc in summer 2001.
(Edit): and obviously a proper keeper in 1999 or 2000.
Naybet had one foot in the door in August 1999 only for the board to nix a move citing fears over his fitness (knee, think it was). They ultimately panic-signed Silvestre two weeks later, who turned out to be a good servant but was clearly not ready to step into a starting role for United. I'm surprised SAF didn't make a move for Thuram between 1999 and 2001, or perhaps he did only for the player not desiring a move to England.
They also signed Bosnich that summer thanks to Edwards. Should have went for Toldo or van der Sar.
Rewriting history would perhaps see something like this:
July - van der Sar
August - Fortune and Naybet
September - Silvestre (for depth/future with Brown, Johnsen, May out for months)
Which means Bosnich and Taibi are nowhere near Manchester.
Or 1999 remains as is but the club signs Rio in summer 2000 window when the need for a top class defender was apparent.