Celtic need to spend more than £50m overhauling their squad this summer if they are to retain their Premiership title and compete in the Champions League, says Chris Sutton. Chris Sutton believes that Celtic's recruitment issues are still a concern and thinks departures of key players could push Celtic into a bigger spend in the summer window. Martin O'Neill led Celtic to their fifth title in a row on the last day of the season in May as a fractured campaign on and off the pitch ultimately yielded a double, with victory over Dunfermline also securing Scottish Cup success. However, whereas Celtic have enjoyed relative dominance in recent years, they were just minutes away from being pipped to the title by Hearts, with managerial turnover and off-field supporter protests over recruitment disrupting last season. Sutton thinks that Celtic are possibly going to have to spend up to or more than £50m really because the squad does need a rebuild. Celtic will start their upcoming Premiership campaign with the visit of Dundee on August 3, and will host Rangers in the first Old Firm clash of the season on September 20. Celtic failed to make the group stages of the Champions League last season, losing out in the play-offs on penalties to Kairat Almaty and Sutton thinks serious work needs to be done over the summer to ensure the club are back at Europe's top table. The squad looked tired last season at times, and Sutton questions whether the squad from last season is good enough to win the Scottish Premiership this season, or good enough to challenge in Europe. Chris Sutton has hailed the appointment of Derek McInnes at Rangers, and thinks that Kris Boyd and he agree that Hearts can still compete.