The Seahawks have done nothing but go backward on their defensive line the past two years — and Jarran Reed’s bizarre six-game suspension has basically completed the retreat to mediocrity.

Over the past two years, the Hawks have lost Cliff Avril, Michael Bennett and Frank Clark from a once very strong D-line. Ziggy Ansah, signed to fill the hole left by Clark, is no sure thing to be ready for the season opener, and now Reed — the top defensive tackle — will miss nearly half of what probably will be his final season in Seattle.

The Seahawks already needed another defensive lineman. Now they need two. And they need to sign them before camp begins this week.

The Reed news explains why Seattle reportedly brought in Ahtyba Rubin a couple of weeks ago. Also available are Corey Liuget and Earl Mitchell, who visited the team earlier this offseason. Expect one of those guys to be signed this week. (Muhammad Wilkerson also is available, but he was arrested for DUI in June so the Hawks shouldn’t be interested.)

Depending on Ansah’s health, it also might be a great idea (as we have said before) to add Nick Perry, who also previously visited Seattle and remains unsigned. The Seahawks have plenty of cap space to sign a couple more guys to a unit that really needs the help.

Barring any additions, the Hawks would end up counting on the likes of Quinton Jefferson, Al Woods, Poona Ford, L.J. Collier, Cassius Marsh and Rasheem Green to anchor the run and provide pass rush. Unless those guys overachieved by a large margin, they would be far from good enough to help Seattle make a strong playoff run (and might be the reason the Hawks did not make the postseason).

Meanwhile, Reed can forget about any kind of preseason extension — and probably any new deal from Seattle at all. Next year some other team surely will pay Reed more than Seattle will want to (which has always been likely anyway).

For now, the Seahawks need to pay a veteran defensive lineman to replace Reed (and another to rush the passer).