Centre-back Rudiger has been out of action since limping off with a knee injury during the 2-0 Premier League defeat to Liverpool on April 14.
Kante, meanwhile, was replaced at half-time of Monday's 2-2 draw with Burnley at Stamford Bridge after sustaining a back injury.
"Rudiger is okay at the moment," he said. "We are trying to recover Kante, but I am not sure at the moment."
Chelsea will also be missing Callum Hudson-Odoi, who had surgery on a ruptured Achilles injury this week, and Sarri expects the winger to be unable to play in pre-season until the end of July.
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"I think Callum will be back here in one or two days. He needs time now, but Callum is a very strong young man, physically and mentally," he said.
When asked how long he thinks the player will be out of action, Sarri replied: "It depends. For sure, three months. Then we have to see the condition, because you can be able to play but we need a player at the top. So, it depends. The technical period is three months, no less."
For now, the return of Rudiger is a boost to a Chelsea side who have conceded seven goals in their last three matches – something Sarri is eager to address against fellow top-four challengers United.
"In the last matches, we had a problem with the defensive phase," he said. "We conceded two goals from set-pieces in the last match [against Burnley].
"It's not only the four defenders. Against Slavia [Prague], we conceded two goals with shots from 20, 25 metres. I think we need to defend with more intensity, more aggression sometimes, and we need to improve in the defensive phases."
Chelsea sit fourth in the table, three points ahead of sixth-place United, making it important to avoid defeat on Sunday in order to keep their Champions League hopes in their own hands.
"In this moment, it's a very important match, probably more than important for our target, so we will try to win of course but we need not to lose," said Sarri.
"It's a very difficult match for both, I think. We have to play against opponents with a very high level of motivation after the Manchester derby. We have to try.
"We have two very important targets at the end of our season: the first is top four; the second is to get to the [Europa League] final. For the first target, this match is very important for us."