Johnson, who was cut after playing just two games last season for the Washington Redskins, had a notable career primarily with the Kansas City Chiefs, where he had two seasons with over 1,700 rushing yards. He has not played a full season for any team since the 2006 season.
In recent years, he has been nagged by injuries, and throughout his career he has had numerous run-ins with the law and has run afoul of team rules, netting multiple suspensions.
This signing does not hearken well for Portis, a nine-year veteran with a recent history of injuries—including a severe concussion in 2009 which cost him the remainder of the season, and abdomen and groin problems during 2010 that limited him to five games and only 227 yards rushing.
Aside from signing former New Orleans Saints running back Reggie Bush, the Dolphins have struggled at the running back position so far this preseason. They recently cut rusher Kory Sheets, and rookie Daniel Thomas is having goal-line problems.
Why the Dolphins chose to take such a gamble by signing Johnson over Portis is unclear. What is clear, however, is that Portis being passed over for the position does not bode well for the running back's chances with any other team.
Considering his recent injury history and limited playing time, combined with the fact that he's not getting any younger, Portis may want to take the signing of Johnson as a hint that he should consider retiring from the NFL.