Sunderland defender Anton Ferdinand has said he still considers himself a fringe player at the club despite a long run in the starting line-up.He told BBC Newcastle: "The squad is stretched. It's up to fringe players like myself to prove their worth. [LNB]"When people come back [from injury] and I'm still playing, and playing well, then I'm not a fringe player." [LNB]The 25-year-old has played a starring role in Sunderland's defence since Michael Turner was injured in November. [LNB]Turner damaged knee ligaments in Sunderland's 2-2 draw with Everton.[LNB] [LNB][LNB] Ferdinand spent the start of the season on the bench but since he made it into the starting 11, the Black Cats have kept an impressive six clean sheets in nine league games. [LNB]There has been praise for the central defensive duo of Ferdinand and Titus Bramble but Ferdinand knows that this partnership will be challenged when Ghanaian international defender John Mensah returns from an ankle injury he picked up last month. [LNB]Mensah is expected to be back in contention in the next four weeks, with Turner not far behind. [LNB]And Ferdinand said he will not consider himself a key player until he keeps these players out of the starting line-up. [LNB]He said: "It's me keeping myself on my toes."