The Scotland international, 21, came off with a knee injury in Monday’s 20-13 loss at Wakefield Trinity.
After a frustrating time with injuries, he had been enjoying his chance with fellow Leeds pivots such as England scrum-half Luke Gale and the versatile Liam Sutcliffe already ruled out for the rest of the campaign.
However, speaking ahead of Saturday’s Magic Weekend game with Hull FC, Agar said: "Callum got taken off with a pretty serious injury.
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“We're awaiting the scans of that but it's likely we'll miss him for the rest of the year.
“We really don't know much more.
“He wasn't feeling great this morning and the patella tendon is the initial thought in terms of the injury.
“The exact damage is unknown at this point."
McLelland, who is out of contract at the end of the season and linked with a return to Castleford Tigers in 2022, has simply not been able to get going this year due to a succession of setbacks.
"I feel for the kid," said Agar.
“He had to be patient waiting for an opportunity and he got some last year, and he would have had more opportunities this season.
“He's had a pre-season and then a whole season that's been punctuated with numerous injuries.
"He's been stood down on Covid and now he's had a cruel way to finish his season as we're coming into the business end of it.
“We really needed him in the halves but he'll bounce back.
“He's still young enough and good enough. He's got a long and fruitful career in front of him."