Jan Vennegoor of Hesselink says Celtic's failure to replace Gary Hooper has cost them dearly in Europe.
Celtic sold a number of key players during the summer transfer window, including striker Hooper to Norwich City.
And former Hoops forward Vennegoor of Hesselink believes their failure to bring in a capable replacement has contributed to their poor results in the Champions League group stages, where they have lost four of their five games and found the net just twice.
The Dutchman told Sky Sports News: "There are good players in that squad but they lost some important ones during the summer.
"If you look at the squad and the resources Neil Lennon has, he did a good job by qualifying for the Champions League group stages, but you can always use a fresh injection of players into your squad.
"They could do with a goalscorer like Gary Hooper. I don't know if Celtic are on the lookout for a striker, but from looking at the Milan game they need an out-and-out goalscorer."
Celtic's 3-0 loss to Milan on Tuesday night, coupled with Ajax's 2-1 success over Barcelona, ended the Scottish champions' hopes of even claiming a Europa League place.
However, Vennegoor of Hesselink believes the experience will aid Celtic's squad and has backed them to respond by claiming domestic league and cup honours.
"Everyone expected more, especially before the Milan game," he said. "They thought they could reach the last-16 or the knockout stages of the Europa League, but with one game left to play their campaign is already over.
"They now need to pick themselves up and try and win the league and cup. They're a young squad and hopefully they can learn from their mistakes."