VERSATILE Tony Carss faces two months on the sidelines after seeing a specialist over an aggravated knee injury.
The news comes as Town prepare for tomorrow's crucial home League I clash with fellow promotion chasers Southend United (3.00).
The much-travelled 29-year-old was one of the candidates to stand in for suspended Danny Adams at left-back as second-placed Town aim to bounce back after taking just one point from two successive games on the road.
Now it seems Jackson will choose between centre-backs David Mirfin and Tom Clarke for the vacancy.
Mirfin, like skilful midfielder Chris Brandon, returns after a one-match ban, while centre-back Martin McIntosh is in the frame after proving his recovery from a knee injury by playing for the Reserves at Barnsley this week.
But for Carss, it's the misery of a stint in the treatment room after an existing problem in his left knee was exacerbated during training.
"Tony has had a bit of pain for the last couple of weeks," reported Jackson, who has fielded the former Oldham man 20 times this season, with eight of his appearances coming as a substitute.
"It worsened this week, so we consulted a specialist who has said he needs rest, treatment, and a special programme of exercises to build the joint back up again.
"It's always a blow to lose a player for any length of time, and with a busy period coming up over Christmas and New Year, it's especially disappointing.
"Tony will be a particular loss because he is so versatile."
Looking towards tomorrow's clash, Jackson accepted: "It really is crucial that we win.
"We are second, they are fifth, and it's certainly the match of the day in this division.
"After drawing at MK Dons then losing at Bristol City last Saturday, it's vital we get back on course in front of our own fans, and we also have the chance to open a seven-point gap on Southend.
"They have had a mini-blip recently (six matches without a win in all competitions), but they are a very decent side.
"They've got an obvious threat in (10-goal top scorer) Freddy Eastwood, who I first saw playing in last season's League II play-off final against Lincoln, when he really impressed me, but they are far more than a one-man band.
"Most people thought a season of consolidation would be on the cards, but they have been challenging at the top, which tells you something."