Steve McClaren's side suffered a number of scares early on against their lowly counterparts, who play in the USL - the English equivalent to League One.
The hosts finished the half comfortably on top and would have gone into the break ahead if not for Krul, who denied Thomas Stewart twice in the closing minutes.
Sacramento had the ball in the net shortly after only for it to be ruled out for offside, but the hosts were given a boost in their pursuit of an equaliser when Haidara was sent off for a foul on Danny Barrera.
Goalkeeper Andy Woodman - who replaced Krul at the break - found himself as busy as his predecessor by producing a series of impressive stops to preserve United's slender lead.
However, 10-man United rode their luck in the end to earn a hard-fought victory.