Benitez sprung a surprise following his appointment as interim manager on Wednesday night by recruiting former Blues and Liverpool star Zenden as an assistant.
The Dutchman had no previous coaching experience but jumped at the chance to help the club where he spent three years as a player between 2001 and 2004.
"It's been really hectic, to say the least," Zenden told Chelsea TV.
"It really came as a surprise to me as well.
"I got the shout at eight, nine-o'clock in the evening, and they told me to come around next day at 1pm at Cobham.
"I thought, 'I'd better start packing my stuff and try to sort out a flight', because it really happened so fast."
Benitez and Zenden were given only three days to pick Chelsea up from their fateful Champions League defeat at Juventus, the latest in a string of poor results that cost Roberto Di Matteo his job as manager.
To make things even more difficult, the new men in charge must somehow try to mastermind a victory over Premier League champions and leaders Manchester City on Sunday.
Zenden added: "The first day was absolutely hectic, a bit of madness, because no-one really knew what was going to happen and everyone has to try to find their feet again.
"It gives you just a short time space to prepare things.
"You cannot go at 200mph - it's not going to work - so it will take a little bit of time.
"The players are willing to pick it up quickly and that's the right attitude, I guess."
Zenden spent two seasons working under Benitez at Liverpool.
"He's very demanding but I think that's what you expect from a boss," the former Holland midfielder said.
"He's someone who was really eager to win games and to pass on the message about his ideas and try to get all the players on board.
"You see that he's a tactical manager, so he likes to implement things and patterns of play and it's up to the players to pick it up - the sooner the better.
"There are so many fixtures and they come so fast that there's not much time to train and the last thing you want to do obviously is - I don't want to say 'bore' them with a lot of tactics - but also you can't work for two or three hours at a time because they won't be fresh again for the weekend.
"So it's a matter of trying to balance it all out and that's why Rafa's here as well."
There were also plenty of other familiar faces at Chelsea for Zenden, including former team-mates John Terry and Frank Lampard.
"I played with JT and Lampard, and all the other players, I surely played against, except a few new ones that just came along this season," he said.
"But, other than that, a lot of people that work behind the scenes, people that are never in the limelight but still do a lot of work.
"It's good to see them again."
Benitez, whose Liverpool links made him a deeply unpopular appointment among Chelsea fans and may also have raised the eyebrows of a few players, hoped Zenden would help in that respect.
"He was a player who was always talking with his team-mates," Benitez said.
"He talks four, five languages and is very professional, very focused.
"So I thought I need someone that, if he knows the inside of the club, he can help with our quick adaptation.
"I think he will help also with our relationship with the players."
Playmaker Juan Mata was pleased with the new coaching staff.
"Not only Rafa but the people who came with him, all of them are high level and Rafa is a great manager," he said.
"He had very good experiences at Valencia and Liverpool so I think we are trying to help him and he is trying to help us, so we are together and trying to win.
"We are training in a different way, we are training more on movement, tactically, and it could be better for us trying to press and trying to play as a good shape."