De Jong, on-loan from Sevilla, struck in victory over Levante on Sunday.
Koeman said: "Luuk's signing was at the last moment to reinforce us in attack. It also depends on the attacking players who are injured. He gives us a lot, he is a different attacker. The other day was good because he was important for the team. He also has experience playing for big teams."
On Champions League opponents Benfica, Koeman also said: "We expect a difficult game, they play at home. There will be a great atmosphere. They have great strength in the games they leave at home.
"Their mentality is to control the game and have a strong defence."
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