Gregan signed a three-year deal today worth £500k, rising to £1m dependant on appearances and promotion.
"You can see that just from walking about the place, with the training ground, the set-up and being around the city centre. I knew it was a bigger club before I signed," said Gregan.
"Looking at Leeds, it's realistic to say we could be playing them in the Premiership next season. There's no reason why it can't happen.
"It would be an ideal scenario. The manager believes he can get Leeds promoted, and so do I, otherwise I wouldn't have come here.''
Kevin Blackwell has spent all summer chasing Gregan as he said: "He's been top of my list since I came here.
"But Sean's lucky. This isn't a step backwards for him, it's a step forward because this is such a big club where we're still averaging 30,000-plus crowds every week.
"It's like playing in the Premier League anyway, and I think that's where Sean Gregan should be playing.
"We may not be the same status as West Brom, but we're bigger in stature.''
Leeds will pay £500k up front plus another £250k dependent on appearances, while another £250k will become due if United win promotion back to the Premier League.
"I wish Sean all the best at Leeds United," said Albion boss Gary Megson.
"He is joining a fantastic football club. He played a huge part in this club winning promotion to the Premiership and was one of the main reasons we did as well as we did towards the end of last season.
"We wish him all the best and I hope he enjoys the same success with Leeds."
"You can see that just from walking about the place, with the training ground, the set-up and being around the city centre. I knew it was a bigger club before I signed," said Gregan.
"Looking at Leeds, it's realistic to say we could be playing them in the Premiership next season. There's no reason why it can't happen.
"It would be an ideal scenario. The manager believes he can get Leeds promoted, and so do I, otherwise I wouldn't have come here.''
Kevin Blackwell has spent all summer chasing Gregan as he said: "He's been top of my list since I came here.
"But Sean's lucky. This isn't a step backwards for him, it's a step forward because this is such a big club where we're still averaging 30,000-plus crowds every week.
"It's like playing in the Premier League anyway, and I think that's where Sean Gregan should be playing.
"We may not be the same status as West Brom, but we're bigger in stature.''
Leeds will pay £500k up front plus another £250k dependent on appearances, while another £250k will become due if United win promotion back to the Premier League.
"I wish Sean all the best at Leeds United," said Albion boss Gary Megson.
"He is joining a fantastic football club. He played a huge part in this club winning promotion to the Premiership and was one of the main reasons we did as well as we did towards the end of last season.
"We wish him all the best and I hope he enjoys the same success with Leeds."