He struggled through the first six holes - "and I even managed a couple of good shots" - but the pain was too much and he was whisked to hospital to check out the damage.
The diagnosis was a deep-seated bruise and Shearer's tournament was over.
"I would rather play than not, but I'm still happy to be back here," said Shearer who was attending last night's tournament dinner boosted by a few painkillers, and will be watching the young guns in action at Belmont today.
It was a nostalgic return for Shearer, who flies back to his home in Victoria tomorrow.
"It is the only place where I won a tournament and had never been back.
"I was driving into the city on Thursday and I didn't remember a thing about it, but then we turned into the golf club and I recognised the entrance.
"And I remembered a few of the holes, though in 1978 there were a lot more trees - big pine trees."
Another big difference was the weather. Shearer beat off Britain's Brian Barnes and American Scott Simpson in driving rain and a gale-force wind 33 years ago. Yesterday conditions were nigh on perfect for professional golf's return to the city.
A life member of the Australian PGA, Shearer had nothing but praise for the Wanganui course. "The course was brilliant and the greens were fantastic," he said.
He gave up competitive golf a few years ago after playing on the seniors tour in Europe.
It brought the curtain down on a career that saw him go head-to-head with Jack Nicklaus to win the Australian Open in 1982 and play in four United States Masters, four US Opens and finish tied for seventh in the British Open of 1978.
"You have to practice to perform and, after 30-odd years of practice, I decided I'd had enough."