When Brian Orakpo re-injured his left pectoral muscle last Sunday in St. Louis, backup Rob Jackson was ready.Jackson said this week he knew Orakpo could be lost at some point this season and, if that happened, his number would get called.On Sunday, that scenario could play out at FedEx Field. With Orakpo on injured reserve, Jackson is listed as the starter in Orakpos usual spot at right outside linebacker. If Jackson does indeed get the nod, it will signal the first career start for a special teams regular who was drafted by the Redskins in the seventh round five seasons ago.Ive been preparing myself since preseason to go in and start, Jackson said. Raks pec has never been right ever since the injury happened last year. So Ive been preparing for a situation like Sunday, when Rak went out and I had to step and play most of the game.After Orakpo went down against the Rams, the 6 foot 4, 266 pound Jackson played 40 of the teams 71 defensive snaps and was credited with one tackle.But that doesnt mean the job belongs to Jackson for the remainder of the season. Coach Mike Shanahan made it clear earlier this week that Jackson and another reserve, Chris Wilson, will battle for playing time. And defensive coordinator Jim Haslett said those two will be pushed by Markus White, who returned to the Redskins after a brief stint on the Buccaneers practice squad.Jackson and Wilson are both considered to be good pass rushers and solid against the run. The player who claims Orakpos job on a permanent basis likely will be the one whos best in pass coverage.They both have a lot of ability, Shanahan said. It will be interesting to see who wants to the job. Jackson added: I think Im ready. I think Chris is ready. I think Markus is ready. Were ready to fill in and step up our game.That said, Jackson has waited four long years to finally get his shot a securing a starting job in the NFL and he doesnt intend to let this opportunity slip easily.I would like it to mine, down in and down out, he said. Thats what you play for. You want to play your best, and I would hope my best would show Im capable of being the No. 1 guy down in and down out.