But Rees admits the heavy defeat at Kingspan Stadium will give much of the Ospreys squad plenty to ponder before Munster's visit.
"We can't hide behind that performance, it was unacceptable," he said. "We were rudderless and we went into our shells and that's to Ulster's credit.
"It's disappointing and unusual in an Ospreys side. I've rarely seen it, if ever, but we've got to play smarter.
"You want to move on quickly. There's not much to take in a positive light but you've got to use it as ammunition to put things right for this weekend and a big game at the Liberty."
The bad news didn't end with the result for Ospreys as centre Ashley Beck suffered a ruptured cruciate ligament and is now facing a lengthy lay-off.
"Initial investigations have confirmed the injury as an ACL rupture and Ashley will see a knee specialist this week," said Ospreys' medical performance manager Chris Towers.
"All being well he will undergo surgery before the end of the week. His rehab will then begin immediately. For this kind of injury we are looking at an estimated eight-month recovery period for him to be back to full fitness."
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