S&P 500 'in a world of pain' so sell any rally: Technician

Stocks are off to their worst ever start for a new year. But if investors think the sell-off is a buying opportunity, a top technician has one simple message for them.

No way.

"The calendar may have changed but the game remained the same, and that game is China…it's the single most important macro proxy we have" said Rich Ross of Evercore ISI on CNBC's "Fast Money" recently. This week, investors ran from a swooning Chinese stock market and a sharp fall in the yuan, China's currency.

In the past year, the S&P 500 Index and the yuan have traded together. But the Chinese government's decision to devalue its currency in August sparked a torrent of selling in the U.S. as investors feared a weaker Chinese economy could hurt U.S. corporate profits.

Read MoreChina 'riding a bronco' with its currency

According to Ross, continued weakness in the yuan should weigh on oil, U.S. transports and U.S. equities. Some have even argued it could spark a global currency war, as economies move to devalue their currencies in order to compete with cheaper Chinese goods.

"That's why you care about the currency, that's why you care about China" said Ross.

The transportation stocks are off 24 percent since its February 2015 high. "This is not a good sign for the broader markets" said Ross.

By Ross's chart work, the decline in the Chinese currency, along with the shift in investor sentiment could spell even more trouble for the S&P 500 Index. According to Ross, if the S&P 500 breaks below its key support level of 1990, it could retest the lows of 1900. "Even if there's a bounce, fade all rallies" said Ross.

At 1900, Ross expects the index to find support, as it would bump up against its 150-week moving average.

"I think we're all in a world of pain here. At a minimum, I think we test that 1900, we can cross that bridge when we get there" said Ross.