Hibbard looked set to replace skipper Matthew Rees in the World Cup squad after the Wales skipper suffered a neck injury, but the Osprey hooker suffered an ankle blow himself ruling him out of the competition.
The 28-year-old has started eight times in the RaboDirect PRO12 this season but has only started four times in his 15 appearances for Wales and has been overlooked for the Six Nations, with Gatland picking fit-again Rees, Ospreys team-mate Huw Bennett and Ken Owens.
Johnson admits he has a soft spot for Hibbard but insists his physical fitness lets him down and is likely to be reason he has missed out on a place in the Wales squad.
"There's a side to Richard which we love and there's a side that lets him down," said Johnson.
"And he has got to understand there are physical requirements all of the time and he has to acknowledge that all of the time.
"To compete with opponents who are up against you all the time you have to got to make sure that you are physically at 100 per cent.
"Richard sometimes can ebb and flow with regards that.
"As a rugby player he should be in the team. Physically, sometimes, he lets himself down.
"And he wouldn't mind me saying that because I'm on his back all of the time."
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