Worst 5 stocks of the week. Here's how to trade them

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U.S. stocks had a tough five days with the Nasdaq posting its worst weekly loss this year.

Below CNBC Pro highlights the worst-performing stocks along with the Street's sentiment to determine whether the downward momentum will continue or the stocks will rebound.

(The price change was calculated as of Friday morning so it is subject to change. If one stock led multiple indexes, we profiled the second best too.)

Worst in the Dow: American Express (AXP), -6 percent this week:

Shares of the credit card company fell this week as investors remain concerned over its ability to replace lost earnings from the expiration of a 16-year relationship with Costco. AmEx stock is down 16 percent this year, making it the second worst Dow component in 2015.

While the Street expects revenue to be impacted by forex and co-branded losses, several analysts believe those concerns are overblown. Management justified the decision to let Costco go as an opportunity to pursue higher-return businesses. At AmEx's investor day in New York, the company indicated it will aim to mitigate losses by leveraging other products and service initiatives through its strong brand.

AmEx is rated "overweight" by 28 financial analysts, who believe the stock could reach $88 over the next 12 months, or 13 percent from where it currently trades.

"Management said [this week] that the company was retaining a large portion of customers in the already terminated Canadian Costco program through other Amex card products," according to Stephen Biggar of Argus Research. "Based on our earnings forecast, the stock's historical P/E trading range, and the company's improving efficiency and strong capital management, we believe that AXP can trade at slightly more than 16 times our forward estimate, or $92 per share."

Worst in the S&P 500: SanDisk (SNDK), -25 percent this week:

Shares of the flash memory maker tanked 18 percent on Thursday after the company delivered a grim forecast for the year. Reaction to the news led multiple analysts to lower their estimates, noting the disappointing results are company-specific rather than industrywide.

The stock was the worst performer this week in both the S&P and Nasdaq 100, and it's now off 40 percent from its high in December. The consensus on the Street points to further losses.

Wedbush research wrote this week:

"We think SNDK's negative Q1 pre-announcement and that the company has withdrawn 2015 guidance metrics supports our downgrade. The concerns from our downgrade on Jan. 22 have increased as we are now worried that qualification delays could accelerate market share losses and more muted cost-downs from market share losses, lumpy enterprise, and continued price pressure could drive another hefty cut to the Street and our lower estimates."

Worst in the Nasdaq-100 Index: NVIDIA (NVDA), -11 percent this week (Second worst behind SanDisk)

Shares of the graphics card manufactured fell this week after Goldman Sachs downgraded the stock to "sell," citing concerns over the expiration of a six-year license deal with Intel. Despite the losses, the stock remains in the black for the year, up 4 percent compared to a loss of 2 percent for the iShares PHLX Semiconductor ETF. The consensus on the Street points to a cautious outlook, suggesting it may be time to stay on the sidelines until new catalysts emerge.

Worst in the S&P MidCap: Apollo Education (APOL), -30 percent this week

Shares of the for-profit education provider dived this week after revenue fell for the 15th straight quarter. A decline in student enrollment along with weak guidance pushed the stock to a new 52-week low, down 44 percent this year alone.

The recent efforts to reduce costs have not worked either. "Companies can only cut costs so much, but you can't cut your way to prosperity" said CNBC contributor Herb Greenberg of Pacific Square Research.

Worst in the Russell 2000: Sonus Networks (SONS), -44 percent this week

Shares of the network and telecommunication equipment vendor crashed 34 percent on Tuesday, after the company slashed its revenue forecast by a third. Despite the dismal results, Jefferies analyst James Kisner maintained a "buy" rating on the stock. "We still believe in the longer-term story," he wrote, pointing to expectations that Sonus could benefit from the long-term transition to IP-based real-time communications. With the shares now off 59 percent this year, this beaten-down name may be oversold, analysts said.

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