While the Coyotes were open to adding another option to their defense in the waning weeks of the summer, the blue line appeared to be set.

They acquired a partner for Oliver Ekman-Larsson on the top pairing in Niklas Hjalmarsson and beefed up their depth with the signing of Adam Clendening.

But what the back end could look like when the Coyotes debut Oct. 5 is now up in the air since Jakob Chychrun is sidelined indefinitely after undergoing knee surgery Thursday to repair an injury suffered this week during offseason training.

The Coyotes said Chychrun is expected to make a full recovery, and they will provide a further update once training camp begins Sept.14. They did not elaborate on the nature of the injury and declined to share any specifics about it or his recovery, so it’s unclear when, where and how the injury happened and what’s next for Chychrun – who became a social-media hit earlier this summer when he posted a video of an impressive box jump during his training.

This setback comes on the heels of a strong rookie debut for Chychrun, who cracked the lineup as an 18-year-old just months after getting drafted 16th overall last summer. He posted seven goals and finished with 20 points in 68 games, flashing plenty of potential as a smart, offensive-minded defenseman who began to earn more ice time as his play in his own end improved.

Chychrun was the third-youngest player in team history to make his NHL debut; his seven goals tied for the second-most among NHL rookie defensemen, and he was the first rookie defenseman in franchise history to tally a point in his first three career games.

A left shot, Chychrun was expected to remain a regular in the top-six and could have earned second-pairing minutes with the Coyotes anticipating lefty Alex Goligoski shifting to the right side.

How his second season progresses is now unclear. It’s also difficult to say how the Coyotes will respond since a timeline for Chychrun’s return hasn’t been announced.

But if the 19-year-old isn’t ready for the start of the season, the team has options.

Arizona could stay pat; it could keep Goligoski on the left. Kevin Connauton could fill out the third pairing with Hjalmarsson, Clendening and Luke Schenn on the right – although it would still be unusual if the team didn’t carry at least one extra.

The Coyotes could call on a minor-league player to improve their depth, if needed. Kyle Wood, a right shot, continues to develop, while Dakota Mermis and Dysin Mayo are other blue liners in the pipeline. Arizona also signed Andrew Campbell and Joel Hanley to two-way contracts in July.

It’s possible the team explores free agency, as experienced defensemen like Cody Franson and Dennis Wideman remain unsigned. The Coyotes could also cook up a trade; while most blue lines look finalized, a few teams seem to have a surplus.

Vegas is one such team and may be open to moving lefties Luca Sbisa, Clayton Stoner or Griffin Reinhart. Buffalo, Vancouver, Nashville and Detroit also appear to have extra bodies on the back end.

Regardless if Chychrun rebounds quickly or is out for an extended amount of time, the news of his injury is a tough blow for a player who showed immense promise as a rookie and a team that has him positioned as a catalyst for its growth.

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Reach the reporter at sarah.mclellan@arizonarepublic.com or 602-444-8276. Follow her at twitter.com/azc_mclellan.