Mikel Arteta said he had doubts when he became Arsenal boss last December that captain Pierre-Emerick Aubameyang would agree a new contract.
The Gabon striker, 31, signed a three-year deal on Tuesday to end months of speculation over his Gunners future.
Aubameyang, whose previous deal expired next summer, has scored 72 times in 111 Arsenal games, including both goals in August's FA Cup final win over Chelsea.
"When I joined, I wasn't as positive as I was in recent weeks," said Arteta.
"Obviously, financially we were in a position that it was difficult to get where we wanted to get.