Northampton prop Kieran Brookes is set for an extended spell on the sidelines after undergoing knee surgery, Saints boss Jim Mallinder has revealed.
The England international went under the knife after suffering an injury in last week's encounter with Racing 92, but an exact timescale on his recovery was not given.
"Kieran had a knee op on Thursday. It's a big blow for us as he's been playing well all season and will be out for quite a while," Mallinder said following his side's shock 25-23 loss to London Irish on Saturday.
Mallinder did at least see captain Dylan Hartley return at the Madejski Stadium from a seven-week absence following concussion sustained in the clash with Saracens in early November, lasting 60 minutes before being substituted.
The boss said: "Dylan went pretty well as he's not trained much with us, but concussion is now a serious part of our game."
Irish boss Tom Coventry was a relieved man after seeing his side pick up their first league points of the season.
Irish produced a huge shock, a penalty try for a scrum offence three minutes from time, converted by Shane Geraghty, earning them victory.
Coventry said: "I haven't had a lot to smile about. It was a good sign that we could battle it out as we have been rolling over against other sides, but we showed a lot of backbone today.
"It was like Christmas Day today as we showed a lot of Christmas spirit to get there in the end. It was a definite shot in the arm as it's been a tough time and will give us a lot of confidence."
The other Irish tries came from Luke Narraway and Sean Maitland, with Chris Noakes adding a conversion and kicking two penalties as Irish built up a healthy 15-3 lead.
Tries from Ken Pisi, George North and Kahn Fotuali'i, with two penalties and a conversion from Stephen Myler, saw Saints rally to take the lead for the first time at 23-18, but the late penalty try condemned them to a surprise defeat.
Mallinder added: "We never really got into our game, we gave away too many soft penalties and they kept it simple and did the basics better than us.
"The referee went a bit early with the penalty try award, but we've no complaints. We've been inconsistent all season and have not been firing on all cylinders."