Millers boss Steve Evans pulled the plug on Roos' season-long loan deal from the Rams on Tuesday after he signed Bournemouth goalkeeper Lee Camp on a two-year deal.
Roos' short stay in South Yorkshire was littered with errors and after a difficult first five games in between the posts, the 23-year-old was relegated to the bench for the Millers' last two games.
His return to the iPro Stadium was not related to his arm injury, which appears to have been picked up in training on Monday or Tuesday of last week.
He went under the knife on his right arm on Friday and will now begin his recovery in a bid to get his career back on track.
Following his return to Derby, Roos tweeted: "good to be back at @dcfc and see the lads. just gutted i can't train, first have to focus on my recovery ... but will be back soon."
operation went well 🌹 can't wait to work myself back! thanks for all the nice messages ❤️ ✌🏼️ pic.twitter.com/Q316xH0F4J
— kelle roos (@kellsroos) September 4, 2015
