Former rugby league star Williams appeared on the wing at Hamilton's Waikato Stadium on Friday as a second-half substitute and scored two of his team's 13 tries, having started in the centre against Tonga last week.
"He certainly made a statement, didn't he?" Henry said, after seeing his team claim their second successive Pool A bonus point victory.
"When he went on he had a positive performance. He certainly got the selectors thinking."
Henry also highlighted the contributions of two-try wing Richard Kahui and flanker Jerome Kaino as New Zealand warmed up for next Saturday's pivotal group clash against France.
"We got better after starting slowly," he added. "We were better than last week. We played with better structure, and I was pleased with that.
"The opposition wasn't as good physically, but they had good structure.
"We've been working on the breakdown. We just wanted to get better than last week, and I think there were good signs of that."
Skipper Keven Mealamu said: "The big thing this week was that we wanted to improve. We have improved a lot, but we still have a few things we can get better on.
"We have some talented boys who can play in different positions, and we're building smoothly. But next week (against France) is a big step up."
New Zealand shredded Japan at regular intervals with Kahui (2), Williams (2), Mealamu, centres Conrad Smith and Ma'a Nonu, full-back Isaia Toeava, scrum-half Andy Ellis, flankers Kaino and Adam Thomson, fly-half Colin Slade and Andrew Hore touching down.
Slade added nine conversions for a 23-point haul, although Japan at least gave their supporters something to cheer when wing Hirotoki Onozawa claimed a consolation score midway through the second-half that Murray Williams converted.
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