Manchester United unveiled Argentina international signing Carlos Tevez on Monday and said they had an option to keep him longer than his original two-year deal.

Tevez joined the Premier League champions in a complicated deal from West Ham after playing a big part in saving the London club from relegation last season.

"We have a two-year deal and then we also have the right to purchase Carlos at any time within that period for a set sum," Manchester United chief executive David Gill told reporters.

"The fee is set down. We know what we have to do in terms of keeping him here for a longer period."