Chelsea, who drew the first leg at Frankfurt 1-1, are hopeful the 28-year-old can feature if they reach the final of Europe's second-tier club competition on May 29.
"He had a problem with his hamstring, not very serious but he needs to rest for a couple of weeks," Sarri told reporters.
"We can try to recover him for only the final if we get there. I'm not sure if he can, but we can try."
Ruben Loftus-Cheek, who could fill in for Kante again, said Chelsea will not take their foot off the gas despite sealing a top-four league finish and Champions League qualification.
"We won't be relaxed tomorrow, we have the Champions League qualification but we have the chance to win a trophy and that will be stupid to go into the match in a relaxed manner," Loftus-Cheek said.
Sarri echoed the views of the England international but warned his players of the danger posed by the German side.
"I think we have reached the target of the club, so we're very happy. We have to play in another competition. We have to reach the final and try to win," the Italian added.
"It's not easy because we have to play a dangerous team as we saw in the first leg. They're dynamic and aggressive and when they can find spaces, it's a dangerous match.
"You have to play a semi-final against a strong team and it's difficult to be favourite. I'm sure tomorrow they can play at their best... we need to be careful."
(Reporting by Shrivathsa Sridhar in Bengaluru; Editing by Christian Radnedge)
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