Sexton, who starred in last year's RBS 6 Nations, is already a doubt for the visit of the Magners League leaders.
"We will make a decision on Thursday - even if Jonny's 100 per-cent he hasn't trained with us and won't train until Thursday," said Schmidt.
"It will be a risk to put him in as he could pull out again but it's tempting because of the quality of the play he brings.
"It's not ideal and we seem to be shuffling the deck a lot more than I would have liked already.
"Munster have had a pretty good injury count and they've learned things over the last few weeks.
"We are off the pace but we're only four games in and there's a long way to go this season."