The first day of the NFL’s open-negotiating period saw the 49ers part ways with one starting defensive lineman – but not the one who was scheduled to be an unrestricted free agent.

Arik Armstead is staying. DeForest Buckner is leaving.

Additionally, the 49ers reached a three-year contract agreement Monday night with free safety Jimmie Ward.

General manager John Lynch, coach Kyle Shanahan and chief negotiator Paraag Marathe managed to get all but one of their starting defensive players from a year ago under contract for the 2020 season.

That is a win.

The loss of Buckner will be felt, though. Buckner was traded Monday to the Indianapolis Colts for a first-round pick in next month’s draft. The 49ers now own the Nos. 13 and 31 picks in the first round.

Last season, the 49ers relied heavily on Armstead, Buckner and Nick Bosa to play the bulk of the defensive snaps. Buckner, Bosa and Armstead each played at least 75 percent of the team’s defensive snaps.

The 49ers will use a rotation to replace Buckner. One player will not be able to step in and take his place. They also should try to give Armstead and Bosa more rest.

Edge rusher Dee Ford is an important piece of the defense, and the 49ers must limit his snaps to keep him healthy and fresh in order to close out games with his ability to get to the quarterback.

Players such as Solomon Thomas, D.J. Jones, Ronald Blair and, possibly, Kentavius Street and Jullian Taylor could help the 49ers form a rotation on the defensive line to take some of the stress off Armstead and Bosa.

[RELATED: Why 49ers decided to trade DeForest Buckner, re-sign Arik Armstead]

The 49ers ranked No. 2 in the league in total defense last season and No. 1 in pass defense, thanks in large part to the pass rush generated from the defensive line. The 49ers lost one of their best players, but there should not be a significant defensive drop-off in 2020.

It’s unlikely the 49ers can add a starting-caliber defensive player on the free-agent market. But the organization has flexibility with their two first-round draft picks to add at a position of need, such as cornerback.

49ers defensive lineup

DE: Arik Armstead/Dee Ford

NT: D.J. Jones/Armstead

DT: Solomon Thomas/Jullian Taylor

DE: Nick Bosa/Ronald Blair

LB: Kwon Alexander

LB: Fred Warner

LB: Dre Greenlaw

LCB: Richard Sherman

RCB: Emmanuel Moseley/Ahkello Witherspoon

NB: K’Waun Williams

FS: Jimmie Ward

SS: Jaquiski Tartt