Leeds have brought in three players so far this summer, with Costa, Ben White and Jack Harrison arriving.
Douglas knows Costa better than most at Elland Road, as the left-back has played with the winger before at Wolves.
And Douglas is excited about what Costa can do for Leeds next term.
“Helder is a player who I know a lot about. He’s got amazing quality and it’s an amazing signing for the club,” the £3 million full-back (Sky Sports) said. “It’s going to keep everyone on their toes.”
"We need that quality to push everyone to their push their limits.”
Leeds have already agreed to buy Costa on a permanent deal, so the pressure is on him to impress.
Costa’s arrival at Leeds has certainly created the most excitement of all their new arrivals.
On his day, Costa has already proven that he can be one of the most dangerous attacking players in the Championship, during his time with Wolves.
Leeds won their first friendly of the season 5-0 last night against York, but Costa did not feature in that contest.
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