Katsuta retired from Rally Estonia last month after usual partner Daniel Barritt reported back and neck pain following a heavy landing on the fourth stage.
Whilst medical checks confirmed Barritt did not break any bones, he was advised to miss the Belgian round in order to make a full recovery.
The late call-up will mark Williams’ first ever competitive outing in a World Rally Car and he admitted it was an opportunity he couldn’t refuse.
“I got a phone call, and they said, ‘what do you reckon?’,” Williams told WRC.com. “Of course I was going to say yes - I wouldn’t turn it down or anything.
“Obviously it’s a shame that Dan’s been injured like he has been, but I’m here to do a job at the end of the day.”
Video: Takamoto Katsuta on new co-driver
The 26-year-old revealed he has tested with Katsuta on previous occasions. He also explained how working with a top-tier team will be a key learning point on the asphalt encounter.
“I’ve done around five tests with Taka now, so I’ve got an idea of what I’m getting myself into,” he said. “Outside the car he’s a good character. He’s very laid back and a good crack, which kind of makes my job a bit easier.
“It’s just going to be the things in between the stages like the work with the engineers and the team communication and that side of things,” he added.
“Toyota have been bang on and really supportive - everyone’s helped me, like Kaj [Lindström, sporting director] and Jarmo [Lehtinen, event leader].”
Belgium has never featured in the FIA World Rally Championship before, but Williams has a good idea of what to expect thanks to previous experience.
He co-drove for Josh McErlean in Ypres in 2019 when the event featured as a round of the Motorsport UK Junior British Rally Championship with a Peugeot 208 R2.
“Obviously it’s going to be different compared to the little R2,” joked the Englishman. “I think there are five stages which are exactly the same, and it’s kind of a similar layout apart from the Sunday.
“For Taka, it’s more the long-term picture. If he can keep plucking his way towards the podium positions then it’ll be happy days.”
- Full coverage from Renties Ypres Rally Belgium will be available on WRC+ All Live here, including every stage broadcast as it happens as well as key interviews, features and expert analysis from the service park.