One day after firing four assistants — including offensive coordinator Mark Helfrich and OL coach Harry Hiestand — Matt Nagy began assembling his 2020 coaching staff with the hiring of new OL coach Juan Castillo, NFL Media's Tom Pelissero reported Wednesday.

Castillo, 60, brings more than two decades of NFL assistant coaching experience to Chicago, where he'll reunite with Nagy after they teamed on the Eagles staff under Andy Reid from 2008-12. Castillo preceded Nagy — and Reid — in Philadelphia, beginning with the Eagles as an offensive assistant in 1995, with a year stint as tight ends coach and two seasons as defensive coordinator flanking his 12-year run overseeing the Eagles offensive line.

Castillo was last in the NFL in 2018 with the Buffalo Bills, coaching the offensive line and coordinating their run game for two seasons, following four years with the Baltimore Ravens in similar roles. While it would be difficult to argue he brings better credentials to the Bears than Hiestand, Castillo's previous rapport with Nagy almost certainly was a factor in his hiring.

And we'll soon find out whether Nagy goes back to his Reid roots to replace Helfrich, like Hiestand, a first-time colleague of the Bears coach last season. If so, two potential names to consider might be Pat Shurmur, who was fired as head coach of the New York Giants this week following two seasons; and Marty Morninwheg, a fellow former head coach and longtime offensive coordinator who was out of the NFL last season after parting ways with the Baltimore Ravens in 2018.

The Bears announced the firings of the four assistants Tuesday roughly 90 minutes after Nagy and Ryan Pace completed their season-ending news conference, in which the head coach was specifically asked about staff changes and said he was still reflecting and working through the decisions.

Chicago finished a disappointing 2019 campaign that began with Super Bowl ambitions at 8-8 largely because of its steep decline on offense, from a top-10 unit in points scored and third down and red zone efficiency to one of the league's worst.

Castillo's previous experience working specifically with the Eagles rushing attacks, including those headed by dynamic lead backs in Brian Westbrook and LeSean McCoy, will naturally shine the spotlight brightly on his tutelage of David Montgomery and role in rectifying the Bears' broken run game.

But with at least four starting offensive linemen under contract and likely returning in 2020, whether Castillo can coax more from a unit that's struggled with consistency — especially creating movement on the ground — should help determine whether the Bears' problems are rooted more in coaching or personnel. Clearly, the Bears are convinced it's the former.