West Coast's first-year coach was asked seven questions on Naitanui at Thursday's media conference.
The 2012 All Australian was a late withdrawal last week against the Sydney Swans.
Simpson said Naitanui was battling back, groin, and Achilles soreness and that the torrential rain in Perth on Sunday had played a part in the decision to pull him from the side.
Speculation followed on whether Naitanui had, in fact, been dropped.
Naitanui was among the best on the ground against Gold Coast in round 13 with 22 disposals and 34 hit-outs, but he followed that performance with two quiet weeks.
Against St Kilda he had 2 touches and 24 hit-outs and against Fremantle he managed just nine handballs and 15 hit-outs before being rested against the Swans.
Simpson said he was shocked by the week-to-week commentary on Naitanui's form and place in the side.
"I do get surprised when it's 'He's the best player in the competition' to 'Should he be dropped?," Simpson said.
"I don't see that in other players. When someone plays poorly or has a couple of poor weeks and they've been an established senior player, an All Australian, I don't see the criticism come so quick and then it goes away again.
"It's a big rollercoaster that I'm sure Nic takes no notice of, and I don't really either.
"But that's the surprising thing for me. The levels of expectation and love, it's just up and down all the time."
Simpson believed the rest was needed for Naitanui given he was playing for a second straight year off little or no pre-season due to his groin problems.
"We are managing him this year," he said.
"There's no excuse for his performance. We want to continue to put games into him when we can, but at the same time you've got to understand his body and where it's at. We think him having a rest is going to make him perform better."
Naitanui did not play beyond round 17 last year when the coach John Worsfold elected to wrap his star ruckman in cotton wool to prepare for the 2014 season. Simpson said the issues this season were nowhere near as severe.
