こんにちは。
「知名度が低い」は little-known と言います。「知名度が高い」「[有名な](https://eikaiwa.dmm.com/uknow/questions/54342/)」は famous や well-known と言います。
例：
He is an athlete that will be competing in the Olympics, but he's little-known.
彼は今度のオリンピックに出場する[選手](https://eikaiwa.dmm.com/uknow/questions/38543/)ですが、知名度が低いです。
ぜひ参考にしてください。
He has a low name recognition＝彼は知名度が低い
Name recognition＝知名度
The media sheds little light on him＝[報道](https://eikaiwa.dmm.com/uknow/questions/65309/)機関は彼をあまり取り上げない
That German athlete has amazing skills and is bound to win in my opinion, but somehow he seems to have very low name recognition＝あのドイツ人の[選手](https://eikaiwa.dmm.com/uknow/questions/38543/)は抜群のスキルを持っているし個人的には優勝すると思っているんだけど、なぜだか知名度はすごく低いんだ。
He shows a lot of promise but he is not celebrity ....yet.
When we hit mass popularity status in any field or area of expertise...We enter
the realms of celebrity...Or instant recognition of the "personality."
Until you are recognised as a "celebrity" ...then you are still a virtual unknown.
"He shows a lot of promise but he is not celebrity yet."
どんな専門分野や領域でも大衆に人気がでるようになると、セレブレティの世界に入るか、すぐに誰だか認識されるようになります。
「セレブリティ」として認識されるまではまだ誰にも知られていない存在です。
He shows a lot of promise but he is not celebrity yet.
「彼はかなり有望だが、まだセレブリティではない」
>He is still unvalued.
>The athlete is still unloved.
Both phrases clearly indicate that the person is not yet well known.
Other words
a.Rejected
b.Undesirable
c.Disfavored
He is still unvalued.
The athlete is still unloved.
まだあまり知られてない人を意味する表現です。
他にも以下の表現をすることができます。
a.Rejected
b.Undesirable
c.Disfavored
Virtually unknown = little known
Household name = a person or thing that is well known by the public.
"He'd never become a household name, unlike his famous younger brother."
New kid on the block = a newcomer to a particular place or sphere of activity.
"What can the new kid on the block learn from the earlier Democrat's mistakes?
Virtually unknown(事実上無名)
= 少ししか知られていない
Household name(誰もが知っている名前)
= 大衆によく知られている人や物
"He'd never become a household name, unlike his famous younger brother."
(彼が彼の弟のように有名になることはないでしょう)
New kid on the block(新参者)
= 特定の場所や活動範囲の新参者
"What can the new kid on the block learn from the earlier Democrat's mistakes?"
(その新参者は以前の民主党の過ちから何を学ぶことができるでしょうか)
example
"The new athlete on the starting blocks is relatively unknown"
or
"The new face on the scene"
You can use more informal words such as a newbie, the rookie, the junior also but the term is usually unknown.
例文
"The new athlete on the starting blocks is relatively unknown"
スターティングブロックにいる新しいアスリートは、比較的知られていない
"The new face on the scene"
現場の新人
ewbie, the rookie, the juniorといったより砕けた語を使うことも出来ますが、その語は通常知られていません。
He is an excellent athlete but he is publicity shy.
He is an excellent athlete but has up to now kept a very low profile.
He is an excellent athlete but is relatively unknown.
If someone is 'publicity shy', it means that he/she is 'fearful of publicity'. The noun 'publicity' itself means information that is given out to the public about someone or product for advertisement purposes or general information purposes. If an athlete is fearful of being in the limelight, that athlete is 'publicity shy' or likes to keep a 'low profile' and becomes 'relatively unknown'.
So, you may say:
He is an excellent athlete but he is publicity shy.
or
He is an excellent athlete but has up to now kept a very low profile.
or
He is an excellent athlete but is relatively unknown.
publicity shy' は、注目されるのを嫌う人を表します。
'publicity' という名詞自体は、広報や宣伝の目的で一般に提供される情報をいいます。注目されるのが苦手なスポーツ選手は、'publicity shy'（目立つのが嫌い）や 'low profile'（目立つ行動を控える）'relatively unknown'（あまり知られていない）などの言葉で表せます。
次のように言えます。
He is an excellent athlete but he is publicity shy.（彼は優秀なアスリートですが、目立つのが好きではありません）
He is an excellent athlete but has up to now kept a very low profile.（彼は優秀なアスリートですが、これまでのところ目立つ行動は控えています）
He is an excellent athlete but is relatively unknown.（彼は優秀なアスリートですが、あまり知られていません）
おっしゃられている内容は、以下のようにも表現できると思いました(*^_^*)
He is not well known.
「彼はあまりよく知られていない」
He is not so famous.
「彼はそんなに有名ではない」
以上です。
少しでも参考になれば幸いです(*^_^*)
★★大阪カフェレッスン英会話講師KOGACHI
"He is under the radar"
'Radar' is a device used more for detecting things such as boats,planes etc..
in this term it implies not many people can see or have seen what he is doing.
"Up and comer" is a term described a lot in sports, this is someone who is very talented and will make it to the top of their sport one day.
"He is under the radar"（彼はあまり注目されていない）
= 'Radar' はボートや飛行機などを探知するための装置です。この表現は「秘密裏に」「注目されていない」の意味です。
"Up and comer" はスポーツでよく使われる表現です。才能豊かで将来を嘱望された選手をいいます。
Example sentence= The unknown athlete is very close to winning a gold medal.
Example sentence= The little known or less know athlete has a great opportunity at winning his first Olympic gold medal.
Example sentence= Many people thought he was an unpopular athlete but he was still able to win a gold medal.
アメリカ合衆国 
日本 
イギリス 
南アフリカ 
