In treacherous conditions after a downpour ahead of the race, Ricciardo had put his McLaren on the inside of Sainz at Tamburello but slid wide and tagged the Ferrari driver into a spin.
Bottas nudged Ricciardo with his Alfa Romeo as a result of the stack-up with Magnussen able to take advantage of the chaos.
"It was like Christmas," said Magnussen. It's lucky I got through that with no contact and had a good position. A good first lap."
Imola "constantly challenging"
The weather played a large part in the Imola weekend with heavy rain hitting the Italian circuit on Friday before a dry Saturday and mixed conditions on Sunday.
Asked how much the team had been able to learn with regard to the VF-22 with such a lack of stability, Magnussen explained: "Well, what you really want to learn about the car is set-up more, there is nothing that you learned this weekend you couldn’t have learned in any other weekend.
"I learned driving these wet tyres, had some laps on the full wets and inters, the dry, in damp conditions, that sort of experience is great and it's been a fun weekend in that regard because it has been constantly challenging."