Cornerback Will Redmond, a third-round draft pick, won’t play at all his rookie season and instead will revert to injured reserve, the 49ers announced Tuesday.

The 49ers envisioned Redmond as their only player eligible to come off injured reserve, but his three-week practice window closed Tuesday with his reconstructed knee not ready to proceed onto the 53-man roster, general manager Trent Baalke told this newspaper.

No other option exists for the 49ers in terms of activating someone else from injured reserve, seeing how linebackers NaVorro Bowman (Achilles) and Ray-Ray Armstrong (pectoral) and cornerback Chris Davis (knee) sustained season-ending injuries. Wide receiver Bruce Ellington is rehabilitating well from a hamstring tear, but he went on injured reserve before the season opener, making him ineligible to come off IR.

Although Redmond won’t be bailing out the league’s last-ranked defense, Baalke noted that the 49ers may have found a gem in JaCorey Shepherd, who was acquired Oct. 10 and has shown promise as a nickel back. Shepherd made a positive impact as a kickoff returner Sunday, when the 49ers lost their seventh straight game, 41-23 to the New Orleans Saints. A torn ACL sidelined Shepherd last season in his rookie year under then-Eagles coach Chip Kelly.

Redmond tore an anterior cruciate ligament last October at Mississippi State, but that did not deter Baalke from making him to the seventh ACL-recovering player drafted without much, if any, success since 2013. “Very comfortable with where he’s at and very comfortable that he’ll be ready to go come training camp,” Baalke said upon drafting Redmond in April.

The other six ACL players drafted: defensive tackle Tank Carradine (2013, second round), running back Marcus Lattimore (2013, fourth), guard Brandon Thomas (2014, third), cornerback Keith Reaser (2014, fifth), fullback Trey Millard (2014, seventh) and wide receiver DeAndre Smelter (2015, fourth).

Redmond’s only game action came in the final two exhbitions, when he totaled 38 defensive snaps.

The 49ers (1-7) and their last-ranked defense could have used help from Redmond — or anyone. They’ve lost seven straight heading into Sunday’s game at the Arizona Cardinals.

Embattled defensive coordinator Jim O’Neil, who announced Redmond’s IR return, insisted that his unit is showing signs of improvement, even though it is on track to allow the second-most points and yards in NFL history.

“There have been three different coordinators here in three years. I came here to help with the problem or be part of the new staff,” O’Neil said. “I believe in what we’re doing and in Coach (Chip) Kelly. Consistency wins in this league.”

O’Neil’s reference to his string of predecessors — Vic Fangio (2011-14) and Eric Mangini (2015) — was to call attention to the fact his defense is still learning a new scheme.

“You can really start to see it click for individual players. It hasn’t clicked for us as a group yet because we’re still making some mistakes,” O’Neil said. “The secondary, we’ve started a new lineup every week, and just them working each week on communicating with each other, they’ve really grown.”

— Wide receiver Torrey Smith (back) did not practice. He has not missed a game in his six-year career.

— Running back Carlos Hyde, who’s missed two games since spraining his shoulder Oct. 16, practiced in a non-contact jersey.

— Defensive tackle Arik Armstead (shoulder), outside linebacker Aaron Lynch (ankle) and cornerback Rashard Robinson (knee) did individual conditioning on a side field.

— Wide receiver Chris Harper, rather than 2015 draft pick DeAndre Smelter, got promoted from the practice squad, and Mose Frazier was signed to fill Harper’s spot.

The 49ers made room for Harper (No. 14) by releasing Keshawn Martin, who was inactive the past two games since fumbling a kickoff return at Buffalo.