And with Wales set to face England on August 6 in preparation for the tournament in New Zealand, Johnson believes Hook, who has was one of Warren Gatland's shining lights in the 2011 RBS 6 Nations, may be a doubt.
"He thought it was a 'stinger', but I'm not so confident," said former Wales boss Johnson.
"Knowing James, for him to go down and come off the pitch is unusual.
"It might be a bit more than he is saying, but let's hope, for his and Wales' sake, that's not the case."
The 25-year-old will now undergo tests on the injury but it was an unhappy ending to his Ospreys career with Hook, who has a history of shoulder problems having had surgery last season, joining Perpignan.
Hook was already set to miss Wales' game with the Barbarians on June 4 as he is getting married that day, but Gatland will be hoping he will be ready for the warm-up matches with England and Argentina in August.