Gray reached out to Patriots coach Bill Belichick and his teammates to apologize for being late, the sources confirmed.
Belichick has a long-standing rule that if a player doesn't report on time, he doesn't practice that day. It happened earlier this year with cornerback Darrelle Revis, who returned later that day to apologize to Belichick.
The absence did not cost Revis any playing time, as he started the ensuing game against the Chicago Bears. As for Gray, who burst into the spotlight in Sunday's 42-20 win over the Indianapolis Colts with 201 rushing yards and four touchdowns, one source explained that he set the alarm on his phone, but the phone battery died overnight and the alarm didn't go off.
Gray, who won AFC Offensive Player of the Week honors, is currently No. 1 on the Patriots' depth chart of power running backs, and is backed up by newly signed LeGarrette Blount and Brandon Bolden. The power backs complement smaller, shiftier running backs Shane Vereen and James White.
It is unclear how Gray's absence will affect his playing time, but he later tweeted about the incident.
The Providence Journal earlier reported the reason for Gray's absence from Friday's practice.