Gronkowski, who broke his left forearm Nov. 18, returned to practice for the first time Friday, participating on a limited basis.
"He's definitely getting closer," Patriots coach Bill Belichick said on the 98.5 The Sports Hub pregame show. "We'll take it day to day and see how it goes."
This marks the fourth straight game in which Gronkowski has been sidelined. He has 53 receptions for 748 yards and 10 touchdowns this season.
In his absence, the Patriots have Aaron Hernandez (third year), Michael Hoomanawanui (third year) and Daniel Fells (sixth year) at tight end. In last Monday's win against the Houston Texans, Hernandez and Hoomanawanui took the majority of snaps in Gronkowski's absence.
The expectation when Gronkowski was injured was that he would be sidelined about six weeks, and a return Sunday night would have been two weeks early. The Patriots, who are vying for a first-round playoff bye, visit the Jacksonville Jaguars on Dec. 23 and host the Miami Dolphins in the season finale on Dec. 30.
In addition to Gronkowski, the other Patriots inactives Sunday night were defensive back Derrick Martin, linebacker Tracy White, offensive lineman Markus Zusevics, defensive end Jake Bequette and defensive lineman Ron Brace.
The Patriots only had to declare six players inactive because they only have 52 players on their 53-man roster.
The 49ers are in a similar situation. Their inactive players were safety Trenton Robinson, running back Jewel Hampton, linebacker Tavares Gooden, guard Joe Looney, receiver Mario Manningham and nose tackle Ian Williams.