The Blues landed Doucoure in September, for around £20m on a three-year-deal, following Watford's relegation.
"When we started to talk about the transfer window, Doucoure was on the list of the club," Ancelotti confirmed.
"He was not on my list, but we talked together and we both (Ancelotti and Marcel Brands, sporting director) agreed to sign him.
"I think it was, after six months, a really fantastic signing because Doucoure is playing really well.
"Doucoure is really good physically and he's able to recover really fast, really quick.
"He doesn't show problems with his physical condition.
"He has played a lot of games but he is still really good, fresh, and has no problems at all."