Motlop, 28, injured his hamstring playing for SANFL side North Adelaide last month.
Initial scans cleared the Motlop of serious damage, but subsequent tests over the weekend uncovered the tear.
Defender Troy Chaplin said the news was frustrating for Motlop, who has been running on his injured leg for several weeks.
"It's disappointing and the medical staff are disappointed with receiving the news that there was a tear because Motts has obviously been running on it and boxing, so that's only going to add insult to the injury," Chaplin said on Tuesday.
"But the exciting thing for us is that we've got guys like David Rodan and Hamish Hartlett, who played a half in the SANFL on the weekend, coming back and Robbie Gray could play this week … so there are three probably first 18 players who could come back into the team (over the next couple of weeks)."
Gray will need to get through Wednesday's main training session to prove he's recovered from a back/hamstring complaint, while Rodan (knee) and Hartlett (hamstring) are likely to spend another week in the SANFL.
Paul Stewart (broken nose) is in doubt for Saturday's clash with the Brisbane Lions after being cleaned up by West Coast defender Shannon Hurn in last weekend's win at Subiaco.
Hurn was suspended for two weeks over the incident, which saw Stewart taken from the ground on a stretcher.
Port Adelaide was accused of acting outside the spirit of the game when it brought the stretcher onto the ground for the second time in the dying minutes of the nail-biting clash.
Play was stopped for Power defender Jacob Surjan when he injured his neck in an innocuous collision with Eagles' midfielder Brad Ebert.
Surjan, 24, regained his feet after the incident but was taken off on the stretcher as a precaution.
Television commentators insinuated Port Adelaide had called for the stretcher as a ploy to stop the Eagles' momentum and West Coast chief executive Trevor Nisbett called for an AFL inquiry into the incident, which he argued cost his side the game.
Chaplin refuted the claims, saying Surjan's injury was legitimate.
"Surj has had shoulder and neck injuries in the past, so the doctor has showed due care because when Surj went down he said he had pins and needles," Chaplin said.
"Surj is our vice-captain and we were already one down on the bench, so why would we want to lose another one? It gave West Coast time to regroup and rethink, so both teams (had the same advantage), if not West Coast had a greater advantage because they've got an extra player than us and there were still four or five minutes on the clock when it happened."
Surjan will remain on a modified training program this week and is not expected to participate in contact drills at training on Wednesday.
Chaplin is likely to have the job on Lions' skipper Jonathon Brown or boom recruit Brendan Fevola when the two sides do battle at AAMI Stadium on Saturday.
The centre half-back conceded it would be impossible for the Power to shut down both star forwards.
"You can't keep champions down for four quarters, so we've got to try our best to curb their influence as much as possible," he said.
"They're going to kick goals because that's what they're paid to do. It's just a matter of how many. If we can get away with two or three goals for each player then I think that's a win."
