Cole has been troubled by a hamstring problem while Onuoha has a calf strain.
Manager Stuart Pearce believes that the 2 games with hosts Shanghai Shenhua and Kashima Antlers will prove as successful as last year's pre-season trip to Thailand.
"I think all things considered, in this day and age, to spend the full 6 weeks at your own training ground is just too long," he told the official City website. "We needed to break that up and last pre-season went very well with having that week long break to Thailand in there.
"The way we've organised our training so far will help us in China, especially with the heat we've been experiencing in England, which has been a little bit tough to work in.
"Our reasons for the visit to China are pretty straightforward really," he added. "There's a tie-in with 1 of our players being Chinese, Sun Jihai, who represents the national side. Also the fact that I thought last year's trip to Thailand went extremely well and again we'll be facing top opposition.
"Micah Richards and Stephen Ireland did really well last year and forced their way into my thoughts as a result of that tour.
"Another draw for us to go to Shanghai is the Chinese people's love of the Premiership. So if we can show people Manchester City in the Far East it can only be good for our club and hopefully people can come over and watch us or follow us on the TV."