Paul Lambert, who captained Celtic during Martin O'Neill's early years at the club, believes the Scottish champions need between £8m and £10m to strengthen their squad this summer.
Celtic is a winning club, not a development outfit, Lambert told BBC Radio Scotland's Sportsound. The former skipper stressed the need to compete for domestic titles and hold their own in European competition.
He acknowledged the squad lacks the quality needed right now but urged against overreacting. "The squad's not strong enough," he said.
So far, Celtic have only brought in striker Camilo Duran during the transfer window. Midfielder Alex Oxlade-Chamberlain signed a fresh contract following his short-term spell last season.
Several targets have slipped away. Forward Kelechi Iheanacho joined Turkish side Bursaspor on a free transfer, while left-back Marcelo Saracchi, who spent time at Celtic on loan, won't return to Glasgow this season.
Saracchi posted on social media that he and his representatives "did absolutely everything within our power to make a return to Glasgow happen." Centre-half Maik Nawrocki has also departed, joining French outfit Lens.
Celtic's chief executive Michael Nicholson previously revealed the club faces an uphill battle attracting elite talent. He explained Scottish clubs are competing directly with English Premier League and Championship sides for players while navigating agent dealings.
