The Redskins traded down to No. 16 and selected Purdue defensive end Ryan Kerrigan, right with Commissioner Roger Goodell. (Jason DeCrow/Associated Press)

The Washington Redskins entered the 2011 draft with a plethora of needs, but with their defense ranked 31st in the NFL last season, Coach Mike Shanahan & Co. placed the highest priority on pass-rusher and drafted Purdue’s Ryan Kerrigan with the 16th overall pick Thursday night.

Passing on a chance to use the No. 10 pick on Missouri quarterback Blaine Gabbert, who would have filled another need, the Redskins traded down and accomplished two missions.

By swapping the 10th pick for Jacksonville’s 16th and 49th overall picks, Washington added another selection to their slim collection. Then the Redskins pulled the trigger on Kerrigan, who should make an immediate impact as they continue their transition to the 3-4 defense.

As last season showed, despite Brian Orakpo’s talent, the defense sorely needed another consistent pass-rusher to provide pressure off the opposite edge. Orakpo last season posted a team-high 81 / 2 sacks, but the other outside linebackers on the team — Andre Carter, Lorenzo Alexander, Chris Wilson and Rob Jackson, who were learning on the fly after playing line their entire careers — combined for just five sacks.

The whole defense mustered only 29 sacks, which ranked 25th in the league. By comparison, the Pittsburgh Steelers and Green Bay Packers — whose attacks most closely resemble the scheme Shanahan and Jim Haslett envision Washington running — racked up 48 and 47 sacks, respectively.

Both those teams have a pair of outside linebackers who can bring pressure from both edges, can play the run and can drop back into coverage. Last year, Carter wasn’t comfortable playing in space, and although Alexander was better in pass coverage, he also took his lumps as he learned a position unnatural for him.

So Redskins brass believed if they wanted to improve, they had to find Orakpo some help. They believe Kerrigan, a four-year player at Purdue who tallied 331 / 2 sacks (second among division I defenders while he was in college), will be the perfect fit.

“Any time you can line up in a 3-4 and you have two guys that can stop the run and also put pressure on the quarterback, and then you go in the nickel situation, you feel comfortable,” Shanahan said. Kerrigan is “big enough, he’s strong enough, he’s played in the three-point stance before. He can go inside, outside. He’s used to playing with his hand down, so it’s a big plus for us.”

Kerrigan played defensive end in college, but the Redskins are confident that he can make a seamless transition. At 6 feet 4, 267 pounds, he has the perfect size for outside linebacker. And at the combine, he displayed great agility and footwork. When he grilled Kerrigan at the combine and during his individual visit to Redskins Park, Shanahan was impressed by Kerrigan’s comprehension of an outside linebacker’s assignments.

“A lot of times you take a guy who’s played defensive end, been in a three-point stance most of his career . . . and having him do linebacker-related work, usually it takes them a while to get going. But he was very athletic, very agile, and we think he can make the conversion fairly quickly.”

Kerrigan remembers watching Orakpo as a standout defensive end at Texas and continued to follow him as he converted to linebacker in the NFL. Now he hopes to make the same transition while teaming with No. 98.

“I feel like I have the ability to play in the two-point stance,” he said. “Obviously, I have to practice it. But I feel like with my pass-rush skills and my athleticism, I can make the transition to outside linebacker. I’m versatile. I’m good against the run, good against the pass and can make plays.”

What’s next? The Redskins enter Friday’s second round with the 41st and 49th picks and several options. They can package those picks and move up higher in that round. Or, they can take a player with the first, and trade back with the second and pick up a third or fourth-round pick.