The 19-year-old is now contracted to Derby until 2018.
Derby County Head Coach Steve McClaren says he's also proud of the fans, who have been 'superb all season'.
Derby County remain in the Championship after Bobby Zamora fired QPR to victory at Wembley. Find out how every player rated out of 10.
Derby County midfielder, George Thorne, has posted on social media of his devastation that he will miss up to 9 months of football action with an injury.
Obviously gutted doesn't really cut it, but I know how to handle it. Appreciate all the messages guys
Dont forget what the team achieved last year. Exciting season ahead & I'll join them for the run in. Look forward to seeing the iPro rocking
Derby County have been dealt a huge injury blow as new signing George Thorne has been ruled out of action for up to 9 months.
The 21-year-old only joined Derby last weekend after his transfer from West Bromwich Albion, but got injured during his first pre-season friendly against Zenit St Petersburg on Tuesday.
Scans have confirmed he has injured his Anterior Cruciate Ligament in his left knee, and will have to undergo surgery.
Derby boss, Steve McClaren told the club website:
– Steve McClaren on dcfc.co.uk
At the time it looked to be innocuous as there was no-one near George and we thought he had just jarred his knee...
... It is a huge disappointment for George personally and we wish him a speedy recovery.
His team mates have taken to Twitter to send their best wishes:
Gutted for @georgethorne15 massive loss for us! But know he will bounce back again! Get yourself back for the run in bro x
Gutted for you @georgethorne15 hope the operation goes perfect.
Derby County has confirmed signing of George Thorne from West Bromwich Albion.
The 21-year-old midfielder has joined the club on a four-year deal after an undisclosed fee was agreed with the Baggies.
Thorne was identified as one of Derby’s top transfer targets this summer after he impressed during a loan spell with the Rams last season.
He made 12 appearances in total and scored twice.
Head Coach Steve McClaren, said:
We’re really pleased that George Thorne has joined us on a permanent basis.
I am sure the fans will be delighted to welcome him back and we’re equally pleased that he sees Derby County as the right place for him at this point of his career.
He was one of the first players Ispoke about with our Head of Football Operations Chris Evans when he arrived inNovember, and we were well aware of his qualities when he joined us on loan in January.
After being patient in waiting for his chance in the side, he showed what he could do on the pitch. His level of performance was excellent.
Derby County players Craig Bryson and Johnny Russell have taken to Twitter to express their disappointment after their 90th minute play-off defeat to QPR yesterday.
Gutted we never got the result we deserved yesterday but we will be back! Thanks for all the support over the season ?
This will hurt for a while but we know what we are capable of as a squad! we will be back stronger for it! Fans unreal all year! thank you??
The Rams had managed to avoid a shot on target for the entire game, until Bobby Zamora popped up with the winner at the death.
Speaking to ITV News Central after the final at Wembley, Derby County striker Chris Martin says the club will now focus on next season.
– Chris Martin
Steve McClaren has galvanised the club and we've taken a big step this season. The dressing room's gutted but we have a young squad and we will push on next season.
Derby County fans have talked about their pride and their faith in Steve McClaren after losing out on a place in the Premie League to QPR.
Fans were watching the game at the Coach and Horses pub in Chester Green and - despite their disappointment - say they are looking forward to a full season under the former England boss next year.
This was the reaction of fans back in Derby when QPR scored their late winner!
They say it is a disappointing end to what has been an otherwise winning season. The Rams were top scorers in the Championship and achieved their highest finish for seven years.
Despite missing out on a place in the Premier League, fans at the Coach and Horses pub in Chester Green said they are proud of their team's performance under Steve McClaren.