Humphrey is a 6-foot-2, 257-pounds rookie out of Central Michigan. He had signed as a non-drafted free agent with Indianapolis April 29. The Colts waived him June 14.
Humphrey fills the roster spot left open by Garrett Cross, who left the team last week.
Humphrey had 68 receptions as a four-year letterman at Central Michigan, where he joined current Packers defensive lineman Cullen Jenkins. The Sporting News selected him second-team freshman All-America in 2001.
Ahman Green was held out of drills during Monday practices with a sore back.
"His back got really tight at the beginning of practice, and I decided to hold him back today," Coach Mike Sherman said. "He's been working real hard. He looks good in practice, but his back stiffened up on him. Instead of putting him through (drills) and because of that, pull a hamstring or something, I decided to keep him out today and get him well for tomorrow."
Other players who were unable to practice, or very limited, included CB's Chris Johnson (shin), Joey Thomas (calf) and Jason Horton (shoulder), RB Walter Williams (ankle), DT Grady Jackson (knee), TE David Martin (groin), WR's Andrae Thurman (broken thumb) and Sam Breeden (hip flexor), and LB Brady Poppinga (hamstring).