The promising 20-year-old was carried off during Wednesday's 2-1 defeat at Everton and subsequent scans revealed a rupture of the anterior cruciate ligament in his right knee.
Thorne spent time on loan at Portsmouth in the 2011/2012 season but was most recently shipped out to Peterborough, where some outstanding displays prompted Baggies boss Steve Clarke to recall him to The Hawthorns.
He was eye-catching in West Brom colours when thrust into the midfield maelstrom against top-of-the-table Manchester United at Christmas and had seemed set to establish himself as a regular before sustaining a similar injury to team-mate Zoltan Gera.
Clarke told the club's website: "This is really sad news - both for George personally and for the club as a whole.
"George is a talented young midfielder who has underlined his potential by doing really well for us in recent weeks. To lose two players in quick succession to such long-term injuries is really tough to take.
"But, as we are already with Zoltan, we'll give George our full support and look forward to seeing him back on the field as soon as possible next season."
The news comes at the end of a tumultuous week for West Brom, whose Nigerian striker Peter Odemwingie courted controversy by trying to engineer a deadline day move to QPR.