Collins arrived in Seattle on Thursday night to begin testing Friday, the source said. The NFL's COVID-19 protocols require six negative tests over six days before a player can enter a team's facility for the first time.
The 26-year-old Collins was drafted in the fifth round by Seattle in 2016, spent the 2017 and 2018 seasons with the Baltimore Ravens and hasn't appeared in an NFL game since then.
The Seahawks are dealing with injuries to their top three running backs. Starter Chris Carson (foot), Carlos Hyde (hamstring) and Travis Homer (knee) all missed practice Wednesday and Thursday, putting their status for Sunday's game against the San Francisco 49ers in question. Friday, coach Pete Carroll said all three are game-time decisions. Hyde is listed as doubtful while Carson and Homer are questionable.
"We're going to have to see how Chris' foot is," Carroll told KIRO Radio 97.3 FM in an interview that aired Friday morning. "We're going all the way to game day on that one. It looks like Carlos has got a hamstring that might keep him from getting in there. So we've got some issues."
Collins has rushed for 1,509 yards and 14 touchdowns on 357 carries (4.2 yards per carry) over three seasons. He made 22 starts over two seasons with Baltimore and rushed for a career-best 973 yards in 2017.
He worked out for the Seahawks late last season and recently for the Detroit Lions.
Information from ESPN's Kevin Seifert was used in this story.