Pitman stood Van Der Werf down from riding at Oamaru and Waterlea on Sunday after she was thrown from a 2yr-old at his Yaldhurst stable yesterday.
"I got bucked off and my lower back is painful," Van Der Werf said.
"Hopefully, I will be back for the final meeting of the season at Oamaru [Sunday week]."
Van Der Werf, who has ridden 27 winners this season, has forfeited the mounts on Heza Kool Kat, Skinydip, Postmaster Bay, Venetian Raider, Trademark, El Santo, Foneeleven and Big Energy today.
David Walsh is the replacement on the Pitman-trained Venetian Raider, the only winner in the race for 2yr-olds. The filly won at Wingatui in May.
Miss Maximuss and All Aboard are the mounts of Chris Johnson, who has helped Pitman compile a tally of 102 wins this season. Miss Maximuss (race 4) was a last-start winner at Ashburton on July 10 with Johnson the rider.
All Aboard (race 10) ran fourth at Washdyke on Sunday.
Pitman has all but ruled out a start for Our Genes in the open 1200m today.
"Unless I find out in the morning that the weather has been bad in Blenheim, he [Our Genes] will go there for the Cup Trial on Sunday. It is a good race for him with only a five-horse field," Pitman said.
Pitman said Milo and El Chico would return to Christchurch next week after campaigning in Brisbane.
Conscious Mistake, who accompanied them, has been turned out there on the property of former Canterbury harness trainer Frank Bennett.
Pitman said he was reducing his team to about 30 horses.
"I am still positive about South Island racing but it is just too tough with all the travelling. I will probably not have an apprentice for the first few months of the new season," he said.
Van Der Werf will undergo surgery on her shoulder at the end of next month and she could be sidelined for six months. Amelia Denby, another apprentice with the stable, will head to Melbourne with her partner, Jonathan Pitman, who will work for an online auction site and complete university studies.
Brittany Moore is back doing light duties for the stable after an injury.
