The 20-year-old will undergo surgery on Thursday and has been ruled out for about 10 weeks after X-rays confirmed the club's worst fears.
Walker limped off at the end of Motherwell's 1-0 win at Tynecastle on Saturday after suffering a knock in the closing stages.
Locke told Hearts News: "It didn't look good for the lad on Saturday and now we know for sure. He's had a scan and he'll have surgery later this week. It's just another blow for us which we'll need to take on the chin.
"It gives an opportunity to someone else, but our options are getting fewer.
"It's the same type of injury which Jason Holt has. It's a massive blow as he's been one of the players giving us a goal threat this season. We have a few out through injury and illness and Jamie's another one now to add to that list."
Midfielder Holt has been out of action since November and might not be back until February while captain Danny Wilson has missed the last three games following a bout of tonsillitis.
Left-back Kevin McHattie came off on Saturday after struggling with a hamstring problem while Callum Tapping and Gary Oliver were included on the bench despite both lacking in fitness following illness and injury.
Hearts have failed in recent weeks to have a transfer embargo overturned and can only apply to the Scottish Professional Football League to sign players on a one-in, one-out basis while they remain in administration.
They are subject to separate sanctions from the Scottish Football Association, which mean they cannot sign a player over the age of 21 until February.