Sam Hutchinson has signed a one-and-a-half year contract with Chelsea just 16 months after announcing his retirement.
The defender, 22, who came through the club's academy, made four appearances for the Blues before a knee injury forced him to quit in August 2010.
However, Hutchinson, who was earmarked as one for the future by former Blues boss Jose Mourinho, realised the pain had gone while coaching at Chelsea.
He has been given the number 27 shirt by manager Andre Villas-Boas.
"It's great to be back and I am looking forward to the future," he said.
"I take it week by week and it is a slow process. I am still not out of rehabilitation but I am playing 90-minute games and feeling fit.
- Chelsea debut:
13 May, 2007 (as a sub, 1-1 draw with Everton)
- Last match:
25 April, 2010 (as a sub, 7-0 win over Stoke)
4 (3 as sub)
16 July, 2011 (played 45 mins of a 6-1 friendly defeat at Crawley)
"I have progressed from a month of three days training and then a day off to four days training and then a day off.
"It is important now to stick to the plan and listen to the doctors and progress from there."
Academy manager Neil Bath added on Chelsea's website: "Should he make progress, we anticipate there could be a further contract but he is still going through the early stages of rehab and is still not training with first-team-type loading in terms of intensity and demands.
"It's fantastic news because we have a player who has been with us since he was seven, who we thought had retired.
"It is looking good that he will have a professional living in the game but we have to stay patient with the plan."
Hutchinson, who can play at centre-back or right-back, made his Chelsea debut, aged 18, against Everton in May 2007.
He signed a new four-year contract at Stamford Bridge in August 2007 but a knee injury blighted his season, limiting him to only six league games for the reserves.
Hutchinson captained Chelsea reserves and made his first start, against QPR in the League Cup, in between a couple of Premier League appearances in the 2009-10 season before the knee injury returned and forced him to announce his retirement.
He made a shock return in the summer and played the first half of a 6-1 pre-season friendly defeat at Crawley.