FOXBOROUGH — Late-season games against division rivals serve as convenient checkpoints, opportunities to take a step back to see how a unit or a player has evolved over the half-season or so since the teams last met.

For Chris Ivory, in the midst of his first full season as the Jets’ starting running back, devolution might be a more appropriate term. He enters Sunday’s game against the Patriots in the midst of a two-month decline that seemingly began the last time the teams played.

To be sure, Ivory is still having a strong year. He ranks among the NFL’s top 10 in yards per game, attempts per game, and rushing touchdowns. But he’s also been trending in the wrong direction, ever since the Patriots eked out a one-possession, 30-23, win over the Jets on Oct. 25.

“I’m not going to say he’s been effective every game from a yardage standpoint,’’ Jets coach Todd Bowles said during a conference call with New England media Tuesday. “He runs hard and he gives us a chance. You always have to bring him down with more than one player, so he gives us a good run-pass option. We’re running the football and try to open things up if you crowd the box too much, so he’s been effective that way.’’

Through the first four games of his season, Ivory averaged 5.54 yards per carry and 115 yards per game. That included monster games of 166 yards and a TD at Miami Oct. 4 and 146 yards and a TD vs. the Redskins Oct. 18.

Then the Jets came to Gillette Stadium on Oct. 25, when Ivory picked up only 41 yards on 17 carries.

In eight games since, his numbers are down: 3.46 yards per carry and 56.25 yards per game. His carries per game are down, too, from about 21 to about 16, but that doesn’t account for the more than 2-yard decrease in yards per carry. Ivory has failed to reach 50 yards in five of those eight games. He’s also had only three TD runs in that stretch.

The reason why is anybody’s guess. Maybe he’s been nicked up, though he hasn’t been on the injury report since Week 10. Maybe he’s just tiring during a long season, with 230 attempts already surpassing his previous career high of 198.

“He’s still an aggressive runner like he was at the beginning of the year,’’ Patriots defensive tackle Malcom Brown said. “Downhill runner. Good cuts, good vision.’’

The Patriots will still have to prepare for him, of course, since he’s the Jets’ lead back. New York also has Bilal Powell and former Patriot Stevan Ridley in the backfield.

“Ivory is a really tremendous back, runs hard, breaks tackles, great balance, very hard to get on the ground, and he’s made plays for them on check-downs and screens in the passing game, too,’’ coach Bill Belichick said. “He’s as hard and physical a runner as we’ve seen this year or probably in the past couple of years.’’

Calling Jackson’s number

The Patriots made official the signing of running back Steven Jackson, who made his practice debut inside the climate-controlled field house.

Jackson, who has rushed for 11,388 yards in his 11-year NFL career, hasn’t played in a game since Dec. 21, 2014, when he was finishing his second season with the Falcons. He was not made available to speak with reporters.

On his first day Jackson already had his familiar No. 39, the only jersey number he’s worn in the NFL. That number had belonged to Montee Ball, another recently signed running back who has been on the practice squad since Dec. 15. But when Ball arrived at his locker, he discovered he was now No. 31.

“For what that man has done and the respect I have for him, I’m more than happy to let him have it,’’ Ball said of the jersey switch. “I wore No. 31 in high school, so I have a special connection to 31, anyway.’’

Seven on Pro Bowl roster

For the seventh straight year, and a franchise-record 11th time overall, quarterback Tom Brady was named to the Pro Bowl. He is joined by six other Patriots, including first-timers Chandler Jones, Jamie Collins, and Malcolm Butler.

Tight end Rob Gronkowski and kicker Stephen Gostkowski are making their fourth appearance each and Matthew Slater was named to his fifth straight Pro Bowl as a special teamer.

The Carolina Panthers had a league-high 10 players named to the Pro Bowl, including quarterback Cam Newton and former Boston College standout Luke Kuechly. The Cardinals and Seahawks also have seven representatives. The game is on Jan. 31 in Honolulu.

Kitchen released

The Patriots released defensive lineman Ishmaa’ily Kitchento make room for Jackson on the 53-man roster. Kitchen signed last Wednesday and was inactive Sunday against the Titans . . . Three players were absent from practice: wide receiver Julian Edelman, cornerback Justin Coleman, and linebacker Jonathan Freeny. The team will issue its next injury report Wednesday . . . Bowles called Butler called “probably one of the best in the league’’ at cornerback. “He’s feisty. He has very good technique, which is rare,’’ Bowles said. “He’s got great hand-eye coordination, he reads the ball, he accepts all challenges. You can tell he works at it. I enjoy watching him play.’’ . . . Belichick spoke glowingly off this year’s crop of Patriots rookies: “As a total group, these guys have been as good if not better than any group we’ve had off the field in terms of their preparation.’’. . . The Patriots had Winter Classic pom-pom hats in their lockers. The NHL’s annual outdoor Winter Classic game will feature the Bruins and Canadiens at Gillette Stadium on Jan. 1.

Michael Whitmer of the Globe staff contributed to this report. Tim Healey can be reached at timothy.healey@globe.com. Follow him on Twitter @timbhealey.