Middlesbrough midfielder Nicky Bailey could be out for up to two months with the knee injury that forced him to miss Monday's 3-0 defeat by Blackpool.
The 27-year-old damaged the medial ligament in the 1-1 New Year's Eve draw with Peterborough, and will miss Saturday's FA Cup visit of Shrewsbury.
"Nicky isn't going to make it, he's longer term," Mowbray told BBC Tees.
"He will be out for anywhere within four and eight weeks, hopefully closer to four rather than eight."
Mowbray added: "He's done fantastically well for us this year, and was evidently missed [against Blackpool] with the style of play he brings to the team.
"Most footballers come across an injury like this in their career, I know I did."
The injury to the former Charlton player ended a run of 26 consecutive appearances, in which he also received a Championship player of the month nomination for December.
Bailey's absence offers Scotland international Kevin Thomson the opportunity to press his first-team claims with a possible first start since October in Saturday's tie with Shrewsbury.
Thomson, 27, suffered a hamstring problem in October, and has since made only three appearances for the Teessiders in that time.
"Kevin had his boots on out there training, so if there's no reaction we hope that he will play some part in the game," said Mowbray.
"It's a fantastic opportunity for Kevin to cement his place in the team over the next four to six weeks.
"Undoubtedly he won't be able to ease his way in because it will likely be blood and thunder. Shrewsbury will come and make it very difficult for us.
"But hopefully he will be able to feel his way into it and be ready for the league games coming."
You can hear the full interview with Tony Mowbray on Friday, 6 January with BBC Tees Sport 95FM and online