Sydney FC defender Stephan Keller has pinpointed the reasons why he thinks the team has fallen from the heights of last season's Premiership.

The Swiss centre-half is of firm belief that the players the Sky Blues lost after their Grand Final triumph over Melbourne Victory have gone a long way to weakening their squad which in turn has forced negative results.

Sydney lost goalkeeper Clint Bolton, striker John Aloisi and defender Simon Colosimo all to Melbourne Heart, midfielder Karol Kisel to his homeland Czech Republic, captain Steve Corica to retirement and youngsters Chris Payne and Iain Ramsay to North Queensland Fury and Adelaide United respectively.

Keller says these losses have done the damage so far this season which has seen the Sky Blues deliver just two wins in 12 matches.

"We let a lot of players go last year and OK, some wanted to go, but I don't think the club wants to make the same mistake again,'' he told the SMH.

''They are all good people who left. We still have a lot of good people here, from the owners, who put in their own money, right down to the fans. But if you include Ramsay and Payne, we lost seven players and that's quite a lot.

''I think of someone like 'Bimbi' [Corica] who was our leader in the dressing room and on the field, who organised everything, and now it's up to other players to do that. He wanted to retire, which is not the club's fault, but more could have been done more to retain others.''

Although Sydney have hit back with two wins and a draw in their past three outings, Keller is well aware that there is still plenty of improvement required, especially from himself.

''I would never be a player to say I'm happy with my performances when you consider the results of the team. It's not about one man or one thing, it's about the team," he added.

''And our team efforts so far haven't been good enough, and I'm part of that.

''We've not been good enough to stand up higher at the level. But I'm not crazy or not sleeping at the moment because of my own performance.''