Despite signing on a permanent basis from Newcastle United during the January transfer window, Williamson is yet to feature for the Championship outfit, remaining on the five appearances that he made during a loan spell earlier in the season.
Henry, who has contributed seven goals in 34 appearances this campaign, has been missing since the middle of February, and while Jackett is optimistic that they will return to the first-team squad sooner rather than later, he has said that they won't be available for the weekend's trip to league leaders Burnley.
Jackett said: "There’s no target date. We’ll have to get them training to see or set a target date. We hoped obviously both of them would be back by now.
“Henry wont be fit for Burnley, neither will Williamson - we’ll see after the international break. Hopefully they can have a good couple of weeks and won’t be far away.
“All I can say is, game by game, they wont be fit for Burnley. After that we’re optimistic, but we’ll have to see.”
Wolves head into the game at Turf Moor in solid form, after previously going seven matches without a win in the second tier.
The Black Country outfit have secured victories over Derby County and Bristol City, while they recorded a goalless draw in the West Midlands derby with Birmingham City on Sunday.
Jackett's team currently sit in 12th position in the standings, 11 points above the relegation zone.