If the Governor knew that Rick and friends would attack at the very moment he put Merle in the arena to fight his brother then that's some Nick Fury level planning.
Who said that? But he knew they were willing to risk coming to rescue Maggie and Glenn, so why wouldn't they come to rescue Daryl who on appearance alone seems like a more valuable asset to Rick from the Governor's viewpoint.
And he didn't seem bothered at all after the attack, strolling through the aftermath of the arena with a smug smile on his face.
It's easy enough to think Governor told Merle to find his way to the prison somehow. He didn't have to have planned everything to work out the way it did for him to want that.