Good news isn’t helping a key biotech bellwether turn its fortunes around. That suggests a gloomy year for the sector.

Celgene, one of the world’s largest biotech companies, announced Monday it expects sales of about $14.6 billion and adjusted earnings of about $8.80 a share in 2018. That’s in line with analyst expectations. But Celgene—and the sector—traded lower on Monday.

Celgene...