Sexton injured a tendon just before Ireland's final Six Nations encounter and will not be available for Leinster's Amlin Cup quarter-final clash with London Wasps on April 5.
And with a move to Racing Metro already sorted in the summer, Schmidt conceded his first-choice no.10 may only be able to return to help his side challenge during the RaboDirect PRO12 run-in.
Ian Madigan continued to shine in Sexton's absence, scoring all 22 points in a vital win over Glasgow Warriors at the weekend to take his side to the top of the RaboDirect PRO12 table.
And Schmidt will be glad he has a more than capable replacement as he was unable to put his finger on a return date for Sexton.
"I'm not sure when Jonny will be back available for us," said Schmidt. "We've been given a five to six week time frame.
"Whatever we are planning over the next five or six weeks will mean it's probably a little bit too soon for Jonny."
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