Manchester United manager Sir Alex Ferguson has confirmed defender Jonny Evans has signed a four-year contract with the club.[LNB] The Northern Ireland international has become firmly established as United's first-choice back-up to centre-halves Rio Ferdinand and Nemanja Vidic and is viewed as a potential captain of the future.[LNB]"Jonny has signed a new deal," confirmed Ferguson ahead of Sunday's seasonal finale against Stoke.[LNB]"We have been working on it for a few months now and I am happy that he has signed it."[LNB]Although the 22-year-old is delighted with the additional security, he realises there is no room for complacency.[LNB]"The contract is there but you still have to perform for the manager," said Evans.[LNB]"If your performance levels dropped you could be sold in January."[LNB]It is a measure of the faith Ferguson has in Evans' ability that two summers ago he allowed Gerard Pique to leave for Barcelona.[LNB]That Pique has gone on to become a regular for Euro 2008 winners Spain speaks volumes in itself.[LNB]And, as a lifelong United fan, there was never any doubt in Evans' mind that he would agree a new deal.[LNB]"Negotiations started a couple of months ago and went pretty smoothly," he revealed.[LNB]"It is a natural process. People think you go into a room, talk and it is done the next day.[LNB]"But these things take time and you just have to be patient.[LNB]"Now it is done and I am really glad to have signed."[LNB]United boss Sir Alex Ferguson has pinpointed Evans as the leader of United's next generation of players.[LNB]Together with the likes of Darron Gibson, Federico Macheda and Danny Welbeck, Ferguson feels the Red Devils have a firm base for the future - even if they relinquish their title to Chelsea at the weekend.[LNB]"Jonny Evans has played a lot of football this year," said Ferguson.[LNB]"For a centre-half at 22 years of age to play so many games and show the quality he has, is tremendous."