The fullback penned a five-year contract yesterday.
Sellars told the club's website: "We think Rayan is a great signing for the football club. He proved last season that he's a very talented player, with lots of potential – I thought he did well and kicked on towards the end of the season. He's a player who is only going to get better, he's a good boy, who works hard, and settled in well here.
“We've used the loan before buying approach well in the past, with people like Jonny and Raul [Jimenez]. I think it's a great way for both parties to get to know each other, to see if they can adapt and enjoy English football, and we get a chance to have a look first, and see if a deal can be done.
“In Rayan's case, he got a lot of minutes because both Jonny and Marcal suffered injury problems last season, so he had lots of games in the Premier League to show his quality.
“At his age, he's got a lot to learn still, a lot of work on, and I'm sure with us he'll develop. He has great quality going forward, we all remember his goal on his debut, so he's got a lot going for him, as well as areas to develop, like all players."