Boeing is running into trouble this week.

The aerospace and defense juggernaut has tumbled more than 5 percent in just two days in one of the worst performances in the Dow.

It now faces a critical technical test, according to Matt Maley, equity strategist at Miller Tabak.

"Looking on a short-term basis, the 200-day moving average has been key support for the stock all year," Maley said Tuesday on CNBC's "Trading Nation." "It did dip below it for one day back in October but bounced back immediately. It's now dipping below it again."

Boeing shares ended right at their 200-day moving average on Tuesday. They have not remained below that trend line for a prolonged stretch in more than two years.

"We'll have to see if it can regain that line rather quickly. If it can't, we will probably see a quick retest of those October lows," said Maley.

While Boeing has seen sharp losses over the past two months, it remains one of the best Dow performers this year, flying more than 18 percent higher. But, therein also lies a problem, according to Maley.

"The problem is that 100 percent of that advance took place in the first two weeks of the year," he said. "The stock is unchanged from Jan. 16 and it's been stuck in this sideways range for 11 months so that's a problem."

Erin Gibbs, portfolio manager at S&P Investment Advisory Services, said recent weakness in Boeing could present a buying opportunity to investors.

"We haven't seen any revisions on earnings or revenue. We're still looking at about 6.5 percent revenue growth for the next two years, looking very solid, and we haven't seen any changes in Wall Street consensus on Boeing, either," she said Tuesday on "Trading Nation."

Analysts surveyed by FactSet hold an overweight rating on Boeing's stock with an average price target of $420, implying at least 20 percent upside.

"For us it just looks like valuations are dropping in, it's an overreaction and this might actually be a good buying point," said Gibbs.

Boeing dropped 2 percent Tuesday on reports the company had failed to properly brief pilots on the features of its 737 MAX jet. That model was involved in the Lion Air plane crash in Indonesia last month.