After being stretchered off in Tuesday night's league encounter with Hull City, it was revealed by the manager that Williamson had sustained ankle ligament damage.
To add to Rovers' woes it is feared that Dickson Etuhu has sustained a re-occurrence of the knee injury that had sidelined him since Christmas.
Etuhu returned to action for the Reserves on Monday, but sustained further injury in training.
"Lee has got a grade two ligament damage in his ankle," revealed the manager.
"It is disappointing from his point of view, he got his opportunity at Arsenal and was magnificent and I thought he did well in midweek before he went off.
"So it is a big disappointment for everyone concerned, he probably is looking at a minimum of a month so that is a kick in the teeth for us.
"Dickson Etuhu has had a bit of a setback. He'll be going for a scan this afternoon.
"We are hoping that he hasn't re-done the same injury in terms of the medial ligament but it looks that way.
"So we've got two midfield players that look like they are going to be out for a month so we are a bit short in that area."
Rovers are back in action on Saturday when Leeds United are the visitors to Ewood Park. Tickets are on sale to those supporters with a database history of purchasing Rovers tickets. Priced from £19 for Adults, £12 Seniors and just £1 for Juniors.
Book online today at www.onerovers.co.uk
and collect your tickets from the Blackburn End collection point on the day of the game.