Chelsea have signed teenage striker Patrick Bamford from Nottingham Forest for a fee believed to be in the region of £1.5m.
Bamford has only played two first-team games but has caught the eye with some superb displays in the FA Youth Cup.
The 18-year-old scored four goals in the last-16 win at Southampton and five in the previous round, but he has rejected two contract offers.
Bamford is out of contract at the end of the season.
"I'm as sad as I was with Wes (Morgan) going. That's a fact of life at this football club at this moment in time," Forest boss Cotterill told BBC Radio Nottingham.
"It's tough times and I just happen to be in charge when the times have got tougher."