Parker may be sound in defence when the defensive line is compressed but when the faster players have room to move in the clear the slowest centre in the NRL is left clutching at thin air.
Speed is a liability is defence
I do recall reading about the sprint trials at Manly one year - I believe it was in the last 3 years - and Brad Parker's results were not slow. In fact I recall reading he was in the top 3 fastest players in first grade, after Tommy and someone else (Garrick?) over 100 metres at the time. Obviously this was before Saab and Koula.
I think calling Parker the slowest centre in the NRL is inaccurate. Tom Opacic (?) at Parramatta is no gazelle, and the Cowboys centres have been nothing flash. I can't recall a true speedster in the centres at the Tigers or Bulldogs recently, and when Staggs is off the field, Brisbane's centres look ordinary.
Does he step and beat players on the outside? Not often. Maybe that's what you're thinking of. Parker tends to step back in off his left foot, and not use an in and away to go around his opposing centre - as Killer used to. Do I wish he was able to frequently do the latter? Yes. However.....
Parker is 100 kgs and 193 cms (that's about 6 foot 4 inches), and strong. I don't think he will ever be the next Phil Blake, Johnny Mays, Johnny Gibbs or Kerry Boustead off the mark. He does a hard job well, has (usually) good reads in defense, he's a local junior, is a team player liked by his team mates, and importantly doesn't cost the club an arm and a leg - so that we can retain the big crop of outstanding talents coming through.
Perhaps in terms of team dynamics, Parker is good value for Manly at the moment, and maybe that's all he needs to be.