Celtic are out of the Champions League and ex-striker Jan Vennegoor of Hesselink puts this down to the inability to find someone to fill the boots vacated by Gary Hooper. The club sold a number of key players during the summer transfer window, including Hooper to Norwich City. Vennegoor of Hesselink believes the lack of 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 scored just two goals.The Dutchman said: "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."Losing 3-0 to Milan last night, along with a 2-1 win for Ajax's 2-1 over Barcelona, put an end to Celtic's hopes of parachuting into the Europa League. However, Vennegoor of Hesselink believes the experience will help the Parkhead squad and has backed them to respond by claiming domestic league and cup honours. He said: "Everyone expected more, especially before the Milan game. 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."