Motlop was sent for scans after injuring his hamstring playing for SANFL side North Adelaide two weeks ago.
The scans revealed no tear, but the mercurial goalkicker is still experiencing soreness in the area.
Gray, 22, developed tightness in his lower back late in the Power’s final pre-season game against Collingwood in Mount Gambier.
Williams said neither player would be rushed back, meaning the star pair might have to wait until round four to play their first games of the season.
“Motts and Robbie won’t play against West Coast. They’ve got a week or two to go and they’re not playing,” Williams said.
The news was better for popular midfielder David Rodan.
Rodan is likely to play a half in Norwood’s league side on Thursday night, just 15 weeks after tearing his anterior cruciate ligament at training.
He has been back in full training for several weeks and Williams said there was no reason to hold the 26-year-old back any longer.
“He’s been training with us for a couple of weeks and he can do everything. We’ve been tackling and dumping him and he’s been tackling and dumping blokes. There’s nothing he hasn’t done except play a game,” Williams said.
“From a mental perspective I think you have to go through a couple of weeks of doing all that stuff at training. Possibly, he could have played [a couple of weeks ago] but for the sake of one or two weeks it would’ve been stupid for us to play him.”
Rodan will be replaced on the long-term injury list by unlucky half-back Marlon Motlop.
Motlop will join his older cousin on the sidelines, after suffering a reoccurrence of a foot fracture that kept him sidelined for two months prior to Christmas.
Motlop’s injury keeps the door open for Port Adelaide to promote another rookie to its senior list.
Rookie-listed recruit Cameron Cloke rucked in the absence of lead big man Dean Brogan when the Power took on West Coast in round two of the NAB Cup and Williams said both Cloke and young ruckman Matthew Lobbe would be considered for selection again this week.
“It’s quite possible we’ll need a second ruckman this week … Cox and Naitanui are pretty good players, so we’ll certainly talk about them [Cloke and Lobbe],” he said.
“I watched Danny Meyer play for Glenelg on the weekend and he did particularly well, so he’s a possibility as well. It’s not like it’s just Cameron or no one. We’ll see and work it out depending on who we’re playing.”
Williams said he also toyed with the idea of bringing promising onballer Hamish Hartlett straight back into the side this week.
Hartlett, 19, hasn’t played since tearing his hamstring at training in January, but is expected to line up for West Adelaide this weekend.
“I think Hamish has only been out one week less than David Rodan, who did his knee, as it turns out,” Williams said.
“We were certainly conservative with him, but he’s trained really well over the last week and should be ready to go.
"We toyed with the fact that he should be straight back in the [Power] side, but we won’t be doing that.”
