Motlop and Archer suffered ACL ruptures that are likely to mean knee reconstruction surgeries and certainly will involve months of rehabilitation.
The bad news from the Blues and Kangaroos came as clubs return to official pre-season training after the Christmas-New Year break.
Archer was injured on Saturday and Motlop wrenched his knee at Monday training.
Motlop's injury comes after he re-signed with the Blues last September on a two-year extension.
In sharp contrast to this year's terrible start, last season he was among only six Carlton players to play every game.
The 22-year-old small forward has kicked 59 goals in 63 games for Carlton since his 2022 debut.
"Jesse has put in plenty of hard work in the pre-season, so we are really feeling for him at this time," Carlton football boss Chris Davies said in a statement.
Archer, son of Kangaroos great Glenn, was hurt one day after his 23rd birthday.
He only managed three senior games last year due to injuries and suspension, for a career total of 26 in four seasons.
Also on Monday, Port Adelaide reported youngster Tom Cochrane suffered a "significant" hamstring strain at training that will most likely mean he misses the start of the season.
It remains unclear whether star St Kilda recruit Tom De Koning will recover in time from a calf injury for round one.
The former Carlton ruckman is back training, but new Saints football boss Lenny Hayes said on Monday there were "a few milestones to get through" before De Koning could be confirmed for the start of the season.
Injury-plagued St Kilda key forward Max King is expected back in training by early next month after his latest knee surgery.
Meanwhile, Geelong's premiership goalsneak Tyson Stengle is taking further time away from the club for personal reasons.
Stengle started his Christmas break earlier in December than his teammates while dealing with an undisclosed matter.
The 27-year-old hasn't since rejoined training with the Cats, saying they hope he will return in the "short term".
"Tyson's welfare is our priority and, with the support of the club, he'll be given the time he needs before returning to training," Geelong's football boss Andrew Mackie said in a statement.
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