Gerken made a string of excellent saves to help City seal the points while Rams counterpart Brad Jones made an error as Jon Stead opened the scoring.
Millen said: "We know Dean is a terrific keeper, he's had to play second fiddle to David James for the whole season and he keeps his head down and works hard so I had no worries about him producing a performance, and he did.
"I was really pleased he kept a clean sheet.
"David understood. Him and Dean get on really well; they work well together in training. He was disappointed he was missing his first league game this year, he's proud of that record but he also understood that Dean deserved a game.
"It's great for me I'm in a position where I've got such great goalkeepers at the club."
Source: PA
Source: PA