The Biarritz player, who had an operation in October, missed France's autumn tour and is unlikely to return until the middle of January.
Betsen was hurt when he was elbowed during in a game against Toulouse on October 9.
However, he should be available for the RBS 6 Nations which starts in Scotland on February 5.
"I was meant to resume training this week but I had complications," Betsen told www.lequipe.fr.
"The doctor has extended my sick leave by three weeks but I hope to be back at the start of January.
"The different experts do not agree between them and that's frustrating. The bruise remains important but I don't have any vision troubles, the only problem is in terms of eye mobility."
France produced a flawless set of results during the November Test matches, beating Australia, Canada, Tonga and South Africa.
And Betsen admits the quality displays of the other flankers has put pressure on him.
"It does put me under pressure but also makes me feel like playing an international match again," he said.
"November has showed our talent and was good ahead of the World Cup. It created a positive competition."