World number one said she was "between sad and disappointed" after pulling out of the Italian Open before Thursday's quarter-finals with injury just a week before the start of the French Open.
"I woke up this morning and I couldn't really move my thumb," the 21-year-old told a press conference.
"I can't move my hand. I can't move my thumb and I'm not sure I can play my match.
"I tried to practice and grip my racket and I just felt this pain every time I tried to move my hand in different directions." The Japanese star was due to meet Madrid Open champion and sixth seed Kiki Bertens for a place in the final four in Rome.
"Right now I'm not mad, but between sad and disappointed," she continued.
"For me it was a test to play against Kiki because she's playing really well and I wanted to see how well I could do.
"I didn't feel anything yesterday. I'm pretty confused when I woke up and couldn't move my thumb, I thought I slept on it and maybe it'll go away but it didn't.
"I haven't seen the doctor the yet. I've seen the physio," she said.
Osaka said she could stay in Rome before going on to the French Open, but did not rule out competing in Roland Garros.
"I'm going to stay here for one more day and see how it is," she said.
"I haven't really talked to everyone so don't really know the plan is yet. I've never had this problem before, it's brand new."
"I'm going to take it like how I went to Australia," added Osaka, who had also retired with a leg injury against Bertens in the WTA finals last October before going on to win the Australian Open.
"Grand Slams to me are like a playground, I have a lot of fun there."
- Finding her feet on clay -
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Osaka won twice on Thursday after the previous day's play was rained off, retaining the world number one ranking ahead of Roland Garros.
She eased into the last eight in Rome with a 6-3 6-3 win over Mihaela Buzarnescu, after earlier beating Slovak Dominika Cibulkova by the same scoreline, as players doubled up after the previous day was washed out.
Osaka had needed to make the quarter-finals to ensure Romanian Simona Halep could not swipe the number one ranking and with it top seeding in the French Open starting on May 26.
But Osaka's top spot was guaranteed when reigning Roland Garros champion Halep fell to Czech Marketa Vondrousova 2-6 7-5 6-3 in the second round.
Osaka, who broke through by winning the US Open last year and Australian Open this year, did not survive the second round in her two previous appearances in Rome.
"I'm sad I've never gone this far here," she said.
"Yesterday the crowd was super nice to me and I wanted to get on centre court to pay them back, but I can't even do that." Before this year Osaka had never made the last eight of any clay tournament, but has now also reached the semi-finals in Stuttgart and quarter-finals in Madrid as she warms up for the second Grand Slam of the season in Paris.
"I would describe (my clay season) as 'rocky'," Osaka said.
"But I mean, I can't necessarily say it's been ups and downs because if I think about it, it's definitely been going up.
"Every match that I've played I've learned a lot. I've tried to take what I've learned into the next match and I think I've done that well.
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