The Japan international is said to be flying to Germany next week for a hernia operation.

The operation would put him out of action for around three weeks, ruling him out of the first match of Celtic's Scottish Premier League title defence, against St Mirren on August 10.

Nakamura missed almost three months of last season with a knee injury and suffered further fitness problems after returning in January.

He played for his country in a series of World Cup qualifiers in June with the help of painkillers but a rest period has failed to clear up his groin problem.

The 30-year-old has not taken part in full training after joining up with his Celtic team-mates in Portugal on Sunday and sat out Tuesday night's 1-1 draw with Middlesbrough in the Algarve Cup.

Nakamura said in the Evening Times: "I've had pain from my groin, an ankle and a calf since the end of last season.

"But the calf and ankle have been getting better in recent weeks.

"While I was playing for Japan, I took pain-relieving medicine. But it got worse."

Nakamura's international team-mate Koki Mizuno is also expected to undergo surgery.

Mizuno came off the bench during Celtic's weekend friendlies against Southampton and Fulham but was left out of last night's game.

He said: "I've been in pain since injuring my knee playing for the reserves against Gretna back in March. It's got so bad I can't play any longer."