Ziyech suffered a twist in his knee in a friendly against Brighton last weekend, while Chilwell was struggling with a heel injury when he signed from Leicester last month.
The Moroccan is almost certainly out of the Premier League opener against the Seagulls, with his return date as yet unknown, while Chilwell is back in training but remains a doubt for the trip to the south coast.
The England international started training at Cobham this week, but has been forced to do so alone due to the injury, so remains some way from being match fit.
‘I’m feeling really good,’ Chilwell told Chelsea’s official website last week.
‘The injury is healing really well and I’ve been doing a lot of fitness work over the past four or five weeks.’
Full-back Reece James is also a doubt for the opening Premier League match of the season after pulling out of the England Under-21 squad this week due to injury.
It is thought to just be a knock and the 20-year-old is unlikely to be out for long, but Frank Lampard does have doubts on both sides of his back four going into the season.
New signings Timo Werner, Kai Havertz and Thiago Silva could all feature.