The Blues had the option to recall the Belgian forward in January but their opportunity to do so expired on Monday, meaning he will remain at the Hawthorns until the summer.
The Blues had inserted a January recall clause in the terms of the striker's deal with the Baggies, but that option expired on Monday.
This means the forward will remain at the Hawthorns until the summer, as he looks to build on the impressive form which has seen him net seven times in 21 appearances.
The 19-year-old helped Steve Clarke's side get off to a blistering start to the season, with the Midlands outfit currently seventh in the table despite a recent dip in form.
The Belgian is set to face tough competition upon his eventual return to Stamford Bridge, with Chelsea having recently purchased striker Demba Ba to battle with Fernando Torres for a starting berth.