He still has a fever, so he hasn't been able to return to work completely.
My coworker had a successful surgery but still has a fever, so he hasn't been able to return to work completely.
he still has a fever (まだ熱がある)
彼は発熱が続いています。
直訳的なhis fever is continuingはちょっと不自然ですね。英語で「まだ熱がある」で表現できます。
he has a persistent feverもOKですね。
My coworker had a successful surgery but still has a fever
同僚が最近手術をし成功しましたが、その後発熱が続いております
仕事には完全復帰できていません。
he hasn't been able to return to work completely.
ご参考になれば幸いです。