Mattock, who has been limited to only three appearances since ankle surgery in December, has a problem with his Achilles and is still troubled by his ankle.
Versatile left-sider Shane Ferguson was able to full in there at Blundell Park, but manager Paul Warne has earmarked a role further forward for the Northern Ireland international signed from MiIlwall this summer.
The Millers' recruitment was anyway expected to step up this week after the sale of Matt Crooks to Middlesbrough for a seven-figure fee was rubber-stamped on Friday.
"It's been evident that we can't rely on Joe to play 50 games every season," said Warne at the weekend. "I'm looking forward to hopefully bringing in a new left-back next week.
"I got told that he felt like his achilles was really tight," Warne said. "He was a bit tired from Tuesday night (when he played in a pre-season friendly at Rochdale). He trained on Friday.
"The intention was for him to play a part. I intended to shuffle things around. But he wasn't fit to join the play. He's been struggling with an ankle for quite a bit now."
Warne has targeted five signings as he reshapes his squad after relegation to League One. He revealed the club has lodged bids for four, understood to include St Johnstone right-back Shaun Rooney and Hamilton Academical defensive midfielder Hakeem Odoffin, but some are either/or options.
Matt Olosunde, a right-back who filled in on the left at times last season, has joined Preston North End and Clark Robertson, a centre-back who can play on the left, has joined Portsmouth. Specialist left-back Trevor Clarke has moved to Bristol Rovers.
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