Skipper Brown was substituted at half-time during Scotland's World Cup qualifier against Wales in Cardiff last Friday night due to his ongoing hip complaint and missed the game against Belgium in Brussels on Tuesday night. Winger Forrest missed both matches with a groin strain.
Neither player is likely to feature in Saturday's Clydesdale Bank Premier League game with St Mirren in Paisley - and midfielder Kris Commons is doubtful for the weekend with a hamstring complaint he picked up playing for Scotland - but the Hoops boss said: "James did his groin in training with the Scotland squad and Scott, as you know, has this ongoing problem."
He added: "I felt he was playing very well against Wales but obviously he couldn't continue after half-time so we are assessing him as we go along.
"We are hoping (he will be fit for Barca), this weekend may be too soon for them but they might have a chance for Tuesday.
"We are still waiting on Georgios Samaras, Emilio Izaguirre, Miku and Victor Wanyama to return (from international duty) but we are hoping to see them later on today or tomorrow but we have no reports of any knocks or niggles."
Lennon denied contract talks with Gary Hooper have broken down and is unimpressed with reports claiming his star striker could be on his way to Liverpool.
With Fabio Borini expected to be out for up to three months with a broken foot and Andy Carroll on loan at West Ham, Luis Suarez is now the only senior striker available to Anfield boss Brendan Rodgers, who is reportedly interested in the 24-year-old former Scunthorpe player.
"It is speculation," said Lennon. "It wouldn't surprise me if there are a number of clubs in for Gary because of the way he is but I would poo-poo that (Liverpool link). We are still in negotiations at the minute and I don't think we are too far away.
"There will be interest in them (Hooper and keeper Fraser Forster) but they are not going anywhere. Fraser has signed a new contract and hopefully we are close to getting Gary's signature. If there are offers coming in, they will have to pay what we feel the valuation is."