ADELAIDE ruckman Reilly O'Brien is in career-best shape after finishing third in the Crows' 2km time trial on Monday morning.

Line-breaking defender Wayne Milera took the honours ahead of fellow backman Jake Kelly, but it was O'Brien's performance that was the major surprise.

The 23-year-old, who played his two AFL games in 2016 but missed the majority of last season with a shoulder injury, is staking his claim to be the long-term successor to lead ruckman Sam Jacobs.

Half-back flanker Brodie Smith and All Australian defender Rory Laird rounded out the top five.

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"It was an impressive time from up and coming big man Reilly O'Brien and some super impressive times from some of the guys we'd expect to excel in this like Milera, Kelly, Smith and Laird," Crows general manager of high performance Matt Hass told the club website.

Crows forward Tom Lynch said the "super talented" O'Brien had the potential to challenge Jacobs for No. 1 ruck spot next year.

"(Jacobs) is a premier ruckman of the competition and has been for a long time and we've got great trust in him, but everyone is always a little bit worried about who's coming in to try and take their spot," Lynch said.

"It's the strength of a great side and that's what we're aspiring to be.

"(O'Brien) has been a man on a mission and it's been great to see him run so well and get some reward for all the hard work he's put in."

It was the first time the entire Crows squad had trained together this pre-season, with the senior players returning on Monday.

Crows captain Taylor Walker was among them at Thebarton Oval, despite having been granted personal leave away from the club after the tragic loss of his and partner Ellie's unborn child last week.

"He's a loveable guy amongst the group and it was great to see him out here training today," Lynch said.

"He's fighting fit, too, after his foot surgery (at the end of last season)."

Midfielder Brad Crouch got through a decent amount of running as he continues to build back up after missing all of last season with groin soreness.

The 24-year-old will be like a new recruit for the Crows, with the club hoping he will be available to play in the JLT Community Series.

"Everyone has mumbled to each other about how good it is to see Crouchy out here today," Lynch said.

"He's a super-elite player and we missed him dearly last year, but as a clubman and a teammate, he's a really good mate of a lot of the guys and it's just great to see him fighting fit and to have a smile on his face."

Veteran David Mackay will return to full training after the Christmas break following Achilles surgery.

Midfielder/forward Jordan Gallucci will also miss a couple of weeks with a toe complaint.