Roos, 23, who came through the ranks at Dutch side PSV Eindhoven, will spend the entire 2015-16 campaign at the AESSEAL New York Stadium, but his long-term future remains with Derby after he penned a new deal until 2018.
Roos’ arrival at the Millers ends a summer-long search for a goalkeeper by boss Steve Evans following the transfer-listing of Scott Loach and the release of Tony Thompson.
The Dutchman, who will join the Millers’ pre-season tour to Scotland, was ever-present on the bench during the Rams’ Sky Bet Championship campaign last year as understudy to Lee Grant, though he did play five times in cup competitions.
Rawson, who joins on an initial six-month deal, is no stranger to the Millers have spent an infamous spell on loan last season.
The 19-year-old played four games for Evans’ side, the last of which came in Easter Monday’s win over Brighton, which he later turned out to be ineligible for.
That resulted in the Millers being deducted three points by the Football League, but they still retained their Championship status.
Boss Evans said: “Since the end of last season I have made it clear that I would like to strengthen the Millers in the goalkeeping department,” said Evans.
“For me there were only two options and I am delighted that we have been able to secure one of them on a season-long loan. I have watched Kelle on a number of occasions, as has our goalkeeper coach Andy Dibble, both of us have on every occasion left the match very impressed.
“I am absolutely delighted to have secured the services of young Faz. In a very short space of time towards the end of last season, he joined us as a boy and left us to go back to Derby County as a man.”