Centre-back Varane went over his ankle midway through the second half in Sunday's 1-1 draw with Chelsea and looked distraught as he made his way into the tunnel at Stamford Bridge.
Ten Hag confirmed that Varane, who won the World Cup in 2018, will miss United's six remaining games before the mid-season break but did not rule him out of the November 20-December 18 tournament in Qatar.
"Rapha Varane isn't in the squad," Ten Hag told reporters before Sheriff visit Old Trafford on Thursday. "He will be out, certainly until the World Cup so he will not play in this block (of games) for Man United.
"I think so (he will be fit for the World Cup), but the prognosis is we have to wait. We have to see how it develops, how the rehab will develop."
Ronaldo was training away from the team towards the end of last week and left out of the squad for Saturday's trip to Chelsea after being dropped for refusing to go on as a substitute against Tottenham Hotspur in their previous match.
The Portuguese re-joined the group on Tuesday, providing a welcome boost for United in attack with Anthony Martial still sidelined after missing their last four games due to a back problem.
"Cristiano will be in the squad tomorrow," Ten Hag said. "It was not difficult. We said everything and we answered all the questions last week. So he was out for one game and now he is back in the squad as usual"
Ten Hag added he will make a late call on the availability of captain Harry Maguire, Donny van de Beek and Aaron Wan-Bissaka, all of whom also returned to training on Tuesday following their injury layoffs.
United are second in Group E on nine points, three behind leaders Real Sociedad, while Sheriff are third on three points with two games remaining.
A point against Sheriff will see United through to the knockout stage, but they need a win to stand any chance of winning their group and avoid a two-leg playoff tie against one of the third-placed teams from the Champions League.
