Gary Hooper and Victory Wanyama are two names who have been linked with moves away from the Scottish champions in recent months.
Norwich City had multiple bids for Hooper rejected during the January transfer window, while Manchester United and Arsenal are thought to be keen on Wanyama.
"It will be a big ask [to keep hold of players]," Lennon told Sky Sports News.
"We will see what the summer brings but there will be players who want to progress their careers elsewhere. We want to bring in young players."
Celtic went down 2-0 in Italy on Wednesday night as they suffered a 5-0 aggregate loss in the last 16 of the Champions League.