Jamie Drysdale
The newest arrival to the blue-line. I don't think his status long-term is in question; Drysdale will be a key member of the core group for many years to come. He hasn't been a Flyer long enough for a meaningful performance analysis, but has impressed so far in his short stint.
Travis Sanheim
Sanheim is experiencing a bounce-back season after a disappointing campaign last year. A rumored blockbuster trade almost sent him to St. Louis in the off-season, but after Torey Krug invoked his no-movement clause, the deal fell through. The Flyers have to be counting their blessings it did. Sanheim is excelling in his new 1D role, left void after the Ivan Provorov deal. He's been a positive at both ends of the ice and is leading all defensemen on the team (excluding Drysdale) with 1.19 points per 60 minutes at 5v5. "Sanny", in all likelihood, will be staying put.
Cam York
Equally unlikely. Although York has experienced a bit of a drop off compared to last year's metrics (only a +0.016 xG 5v5 differential), he is playing top pair minutes and still producing offense at an acceptable clip (0.78 per 60 at 5v5). The Flyers should be in no hurry to move their 2019 first round selection. After all, they passed on Cole Caufield to draft him. He will continue his development as a member of the orange and black.
Rasmus Ristolainen
Just a year ago, "Risto" would have topped this list. A frequent punching bag of the advanced stats community, the big defenseman has long struggled in his own end but produced respectable metrics offensively. This season, however, is a different story. In a turnaround few saw coming, Ristolainen has the best xG differential on the blue-line at +0.631. His offensive numbers are down, but the Flyers will happily accept that if this is the new version they can expect. Briere and co. wouldn't mind shaving off his hefty contract, but he provides size and physicality to an otherwise undersized D corps. There could be some temptation to move him given that he's reportedly highly coveted around the league, but I don't anticipate that happening.
Egor Zamula
The only player with a negative xG differential on this list (-0.086), "Z" is an interesting case. He's putting up decent offensive numbers (0.71 points per 60), but less than stellar metrics in his own end. He is currently the power-play QB on the top unit and succeeding at it, after piloting nearly everyone else to try and spark play on the man advantage. It remains to be seen if he will remain in that position with Drysdale in the picture, but the recent success of the power-play (goals in 5 of 6 games) can partially be attributed to his play. The team is in no rush here, but Zamula is certainly a bottom pair defenseman at this point.
Marc Staal
Firmly in seventh defenseman territory, Staal has seen very limited usage at the halfway point of the season. His underlying metrics are good (+0.539 xG differential), but he's only averaging 12 minutes of ice time per game in his rare appearances. He is likely benefiting from the sheltered role. Regardless, Staal is a veteran and was very publicly brought on to mentor this inexperienced group. It was made clear to him in the off-season that he'd be seeing limited minutes, as the Flyers try to develop some of their younger pieces. Given his role, shipping Staal out does not move the needle any. Tortorella would likely end up having to bench Zamula, who will do nobody any good sitting in the press box. At this stage in his career, Staal is not a fit for a contender so I imagine he will remain as a veteran presence.
Nick Seeler and Sean Walker
Odd to put these two together, I know. But in December, Elliote Friedman reported that at least one team inquired about making Seeler and Walker a package deal. The pairing has been a pleasant surprise for the club. Together, their xG differential is +0.49, with Walker's offensive output (0.9 points per 60) elevating that a bit. Seeler, an established Flyer at this point, plays a hard-nosed and defensive-minded brand of hockey. Walker, originally a contract dump in the aforementioned Provorov trade, is a solid puck carrier and has shown the ability to score in transition. Both players want to stay in Philadelphia, but ultimately I think this pairing will be impacted should a move occur. Walker seems to be the most likely candidate, as he's a right-handed shot and suddenly the team has an influx of those. He has also been made slightly redundant by the addition of Jamie Drysdale, as both players have a similar skill set. It would be a shame to lose a player who has made such an immediate impact and has voiced his desire to stay, but you get the feeling Sean Walker's days could be numbered.