A Nike store in Spain.Xavi Lopez/SOPA Images/LightRocket via Getty ImagesNike just said its sales will fall over the coming quarters, sending its shares down 20%.The sportswear brand has started implementing a turnaround plan, but the results are yet to be seen.Nike's reliance on its lifestyle business seems to be hurting it, one analyst said.Nike is facing one of the most challenging periods in decades.In December, the company announced a cost-cutting initiative that included simplifying its assortment of products and cutting jobs.The aim was to deliver up to $2 billion in savings over the next three years, which "will be invested to fuel future growth, accelerate innovation at speed and scale, and drive greater long-term profitability."Two quarters on, investors aren't seeing much of that growth.On Thursday, Nike delivered a lackluster set of Q4 results and said it expects sales for its 2025 fiscal year to fall by mid-single digits, including a 10% drop in the first quarter alone.