In his 13th AFL season, Stringer started as a sub for the first time and kicked three goals as the Giants went down by 14 points at the SCG.
While the move raised eyebrows, Kingsley said he had started Stringer on the bench in a bid to help the former Essendon spearhead increase his fitness.
Stringer had been kept goalless in last round's defeat to the Western Bulldogs after playing with sickness.
He had been dogged by illness in the pre-season, missing two simulation matches before a hamstring strain forced him out of the first two home-and-away rounds.
"Ever since he got quite crook a week and a half before the opening round, (his preparation) has just been a bit patchy," Kingsley said.
"He got crook again last week and played crook (against the Bulldogs) and so I made an error of judgment there. I thought he'd be OK, but he wasn't.
"We need to get some work into him to get him near his best.
"If that means he misses a week and we use that extra week to train him and get some work into him, then that's what we'll do."
Stringer replaced Toby Bedford after the star tagger was caught high by Sydney defender Lewis Melican.
Melican will face scrutiny after Bedford was forced out of the game with blurred vision.
Kingsley said Bedford did not suffer a concussion.
"He can't see out of that eye at all, and so we had to make the sub at halftime, which was obviously quite disappointing for us," Kingsley said.
"He'd started on (Isaac) Heeney and didn't start particularly well, but he'd shifted to (Chad) Warner after probably five minutes of that quarter and was doing a really good job.
"I don't think Warner had touched the ball and maybe had one for the remainder of the quarter - no doubt it was certainly a little bit crippling for us.
"When it's a really important player to the way that you play structurally, then you've got to go to plan B, which isn't as good as plan A.
"If it was, it'd be plan A."
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