If people get the chance, check out Seahawks.com for the phone press conference video of Avril with the media and also Pete and Johns latest phone press conference.
From what I've gathered listening to those two videos is that Avril is to be the Day 1 starting LEO as Clemons recovers and because Irvin still has a way to grow, though they still really like him as a specialist for now.
For whoever asked above, left defensive end is the LEO position and the position Avril played at Detroit the last two years, though he did play Right defensive end (Red Bryant's position) his first two years there, so he's versatile and does have the skillsets to do some of the drop in coverage stuff that Carroll has had Clemons and Irvin do.
But he made it pretty clear. He's a pass rusher first and unless Irvin beats him out, he's the new starting Leo.