Premier League experience
Sunderland made mistakes in last summer's transfer window, recruiting no less than 13 players from overseas with little or no Premier League experience between them. The end result was a dreadful start to the season which ultimately cost Paolo Di Canio his job, and left Sunderland in a season-long fight for survival.
It is never easy to bed a player in at a new club, but it is certainly easier when the player is comfortable and familiar with the league in which he is playing.
Williams has forged his whole career in English football, and enjoyed two highly successful seasons in the top flight with Swansea City. The 29-year-old should not take too long to settle in, and that will be valuable if Sunderland are to make a good start to the season.
The fact Williams is already Premier League proven also limits the amount of risk in the deal.
Williams is evidently a born leader on the football pitch. Williams captained Stockport County before making his move to the Liberty Stadium in 2008 where he was vice to Garry Monk before taking over the captaincy full time in 2013.
Williams had already at this time been made captain of his country by Chris Coleman, replacing Arsenal's Aaron Ramsey in the role.
Sunderland had a fairly youthful squad last season and were often without real leaders on the pitch. Poyet regularly relied on the experience of John O'Shea, Wes Brown and Seb Larsson to keep order on the field, but this is an area in which the club would definitely benefit from strengthening.
One of Sunderland's major downfalls last season was their physical presence in defensive areas, especially when defending set pieces. On too many occasions Sunderland were undone by a lack of leadership in their own six yard box and again this is an area in which Poyet must strengthen if the club are to progress.
Williams is not the most imposing of centre backs, but he more than makes up for this with his broad and robust build. The Welsh international is strong, composed and commanding with the confidence to also find a pass after retrieving the ball.
Swansea City are expected to resist selling their much loved captain, but Williams has hinted at a desire to leave the club and Sunderland clearly fancy their chances of snaring the player. However, £4million may not be enough with the Swans likely to demand double that before they are prepared to let the player speak to a league rival.