According to the Lions' official website, Detroit would like to hold onto Williams. Williams is one of the better depth defensive tackles in the league, someone who could start for quite a few teams. However, Williams sits behind Ndamukong Suh and Nick Fairley, perhaps pushing him to go elsewhere for a starting gig.
Jackson may be seeing some significant playing time now that Avril has moved on to the Seattle Seahawks for a two-year deal. Last year Jackson amassed 20 tackles and 2.5 sacks in his fifth year out of USC.
Regardless of what the team does with Jackson, the defensive end position still needs to be addressed. This is something that doesn't escape head coach Jim Schwartz, per the article:
"Eventually (we're going to need another defensive end), but there are a lot of avenues," Schwartz said.
"I think every year you’re going to find pass rushers every step of the way," Schwartz said. "There’s going to be pass rushers in the first round, second round, third round ... we talked about Willie Young before, he was a seventh-round pick.
Detroit is drafting fifth in April, so it could take almost anybody it wants along the defensive line.