Daily La Voix du Nord dubbed manager Philippe Saint-Andre's choice as "controversial" while number eight Imanol Harinordoquy's wife wrote on her Twitter feed: "I don't know Claassen but it seems to me that we have what we need in terms of French loose forwards!"
"Of course my case is unusual and I appreciate that some may not be happy," Claassen told Reuters .
"I know that I will have more pressure than the others and that I will be observed more closely."
Foreign-born players have represented France in the past and Claassen follows in the footsteps of compatriot Pieter de Villiers, Algeria-born Abdelatif Benazzi and New Zealand-born Tony Marsh.
"Nothing can alter the joy I have been feeling since Monday. I am proud, it's magical," said the Castres loose forward.
"I was at Twickenham two years ago with my mates to watch England v France. My God, what an atmosphere!"
Claassen, whose father Wynand was a member of South Africa's controversial tour of New Zealand in 1981, came to France in 2007.
He joined Brive from the Blue Bulls and spent five years in Correze before moving to Castres this season.
"I have been in France for five years and I feel better here than in South Africa," the Durban-born Claassen said.
"I don't know how to explain it, it's a matter of sensations. I like living here and I'd like to live here all my life."
When he arrived in France, Claassen was greeted by compatriot Gerhard Vosloo, another loose forward now playing at Clermont.
"He introduced me to (then Brive coach) Laurent Seigne. I played with the Under-21s at first before being called up to the first team and my French adventure started," said Claassen.
Former France coach Marc Lievremont had considered picking him but Claassen had not been resident in the country long enough.
The International Rugby Board states that to be eligible for a national side a player must be born in the country, have a parent or grandparent born in the country, or have spent three consecutive years of residence immediately preceding the time of playing.
In Brive, Claassen would invest in properties to be restored with Vosloo and gradually put some roots in France.
"When Brive were relegated (in the second division), I cried," he said. "I was so sad for the players, the fans, the city."
Claassen still has a strong South African accent, but speaks almost perfect French.
"I learnt the language and the traditions, but it was not really an effort, more a sign of respect," added Claassen, who says he knows the Marseillaise by heart.
When he sees his son wear the France jersey at Twickenham on February 23, Wynand Claassen will probably be a proud man.
"I think he will be emotional. He's the one who paid for my ticket to France in 2007," said Claassen.
France lost their first two six Nations games against Italy in Rome and at home against defending champions Wales.