Celtic manager Ronny Deila hopes to find new players in time for the first leg of their Champions League play-off against Maribor.
Last season's Scottish title winners have been reinstated to the competition after Legia Warsaw were punished for fielding an ineligible player.
Legia beat Celtic 6-1 on aggregate in the third qualifying round.
And Norwegian manager Deila says Celtic are "working really hard now to get players in for the next stage".
The first leg of the Maribor tie will take place in Slovenia on 20 August, with the return at Celtic Park six days later.
The winner will progress to the Champions League group stage, with the loser parachuted into the Europa League group stage.
"Of course, there are many aspects of keeping players and getting players in but that [Champion League football] is one of those aspects.
"It is a vital thing [getting back in]. It will lift the team and performance."
The Scottish champions' transfer kitty is expected to be boosted by the proposed sale of goalkeeper Fraser Forster to Southampton.
|Celtic sales in last three years|
|Islam Feruz, Chelsea, £300,000||Jos Hooiveld, Southampton, £1.5m|
|Gary Hooper, Norwich City, £5.5m||Denny Johnstone, Birmingham City, £300,000|
|Efrain Juarez, Club America, £1.9m||Joe Ledley, Crystal Palace, undisclosed|
|Shaun Maloney, Wigan, £850,000||Morten Rasmussen, FC Midtylland, £250,000|
|Ki Sung-Yeung, Swansea City, £6m||Victor Wanyama, Southampton, £12.5m|
|Tony Watt, Standard Liege, £1.2m||Kelvin Wilson, Nottingham Forest, £2.5m|
A £10m fee has been agreed, of which Forster's former club Newcastle are entitled to a percentage, and the England cap will undergo a medical on Saturday.
Should the deal be completed as expected, Forster will become the latest high-profile departure from Celtic Park.
Celtic, who have signed goalkeeper Craig Gordon and forward Jo Inge Berget this summer, took in over £20m from the sales of Gary Hooper, Victor Wanyama and Kelvin Wilson last year, having sold Ki Sung-Yeung to Swansea for £6m in 2012.
In January midfielder Joe Ledley moved to Crystal Palace for an undisclosed fee.
Deila, who replaced Neil Lennon this summer, said after Tuesday's meeting with Legia that he wanted "players with pace, ambition and who are young".
"I want to build a team who could last for many years," he explained. "It is my responsibility but I need time to get players in.
"I saw things which were good [against Legia] but there were too many things that need to improve.
"I have to accept that is the level right now and have to work from here.
"I have been here for six weeks but right now it is not good enough, from what I can see."