United seemed to get a bit nervous after the Baggies had pulled one back after the Reds stormed into a 2-0 lead.
Van der Sar spilled a cross from Somen Tchoyi which gifted him an equaliser.
"Edwin van der Sar has probably been playing for 25 years and was probably in primary school the last time he made a mistake like that," said Ferguson.
"You have to just wipe it off the slate."