Shane Duffy will miss his first match of the season this weekend.
After starting every league game this campaign, the Irishman suffered a groin strain during Tuesday night's 1-1 draw with QPR, and whilst Rovers still await the results of a scan, he will miss Saturday's match against Brighton & Hove Albion.
"He's in high spirits at the minute but we are still waiting for the results of the scan," said Lambert on Thursday.
"With a bit of luck he won't be out for too long, but I'm pretty sure he won't make an appearance on Saturday.
"It is a blow because he has been playing well."
With Matt Kilgallon also struggling with a hip complaint, that leaves the Rovers boss with just one fit central defender - club captain Grant Hanley - ahead of the Ewood Park clash with Brighton, which has prompted Lambert to look to the loan market for defensive reinforcements.
"The centre-half area becomes really important now with Duffy being out," added the Scot.
"I don't know how long he'll be out, but I always think that you've got to have four centre-halves at your football club and at the minute we've only got three, and two of them are injured.
"So the centre-back area becomes a big area for us, as it stands right now at the minute we've only one fit centre-half.
"I have to try and get somebody in, I've made a couple of calls already to see if somebody is available."
Tickets for Saturday's match against Brighton will be on sale up to the 3.00pm kick-off with prices from £18 for Adults, £12 Seniors, £12 18-21-year-olds and from just £7 for Juniors aged 17 and under.
Why not avoid the matchday queues by booking online at www.eticketing.co.uk/onerovers and collecting your tickets on the day of the game.