The Everton boss is now in charge of managing the Icelandic playmaker after he completed his £45m switch from Swansea on Wednesday.
"In my opinion, in his position he is one of the best in the Premier League. We have the player here and that's really good for the team," said Koeman.
"From the beginning, he was one of the key players we wanted to bring to Everton. That's because of his quality, first and foremost, and his experience in the Premier League.
"We need more productivity. We don't sign one player who will score 25 goals but we need several players around our striker to get that number.
"He is one of those productive players which he has shown at Swansea and also in the past."