The 23-year-old Japanese played just nine holes at the Riviera Country Club on Wednesday and was unable to practise at all on Tuesday, spending most of the day resting.
"I'm on medication now and all I can do is rest and drink lots of liquids," Matsuyama, a double winner on the PGA Tour who is one of the game's most exciting prospects, told reporters via an interpreter. "Hopefully I will be better tomorrow.
"I'm striking the ball well. I just really need to get some rest and get healthy so I'm ready to play tomorrow."
Matsuyama is certainly very comfortable with the par-71 layout at Riviera, having tied for fourth at last year's event after closing with a four-under-par 67.
He is also in superb form, following a gripping playoff victory over American Rickie Fowler at the Phoenix Open earlier this month before he decided to skip last week's PGA Tour event at Pebble Beach.
"It was an off-week last week for me, so I got my team together and we did celebrate," smiled Matsuyama, who has also won six times on the Japan Tour.
When asked if those celebrations might have contributed to his flu-like symptoms over the past 24 hours, he swiftly replied with another grin: "No, no, it has nothing to do with the celebration."
Despite not feeling at his best, Matsuyama was delighted to be back at Riviera for his third appearance at the Northern Trust Open.
"Riviera Country Club is always in pristine condition," said the world number 12. "The greens are very good, very fast, and you have to play well here.
"You have to strike the ball well to play well here, and that's one of the reasons I like to play here. I have great memories here at Riviera. I just hope I'm feeling better tomorrow, so I'll do my best."
Matsuyama has been grouped in a high-profile pairing for Thursday's opening round alongside Northern Ireland's world number three Rory McIlroy and American Matt Kuchar, a seven-times PGA Tour winner.
(Editing by Steve Keating)
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