Northampton player-coach Andy Holt has suffered a suspected dislocated collar bone in training and is expected to be out for six weeks.
The 33-year-old had been stepping up his pre-season regime following knee surgery in the summer.
"It's a blow for Andy," boss Gary Johnson told the club website.
"It was an innocuous challenge with one of our goalkeepers, he just fell awkwardly and we suspect he will be out for around six weeks."
Johnson, who had been expected to use the utility man at centre-half this season, added: "He was doing really well recovering from his knee operation, and then he picked up this injury.
"It's a blow to him and us, but he has at least got some time to swot up on his coaching manuals."
The Cobblers do have defensive cover in Byron Webster, John Johnson and Ashley Westwood, who is at Sixfields on non-contract terms.