And the 25-year-old made an immediate impact as he scored a try in their 34-20 win over Newport Gwent Dragons in front of a Guinness PRO12 record crowd of 68,262 at the Principality Stadium.
"I didn't expect him to go that well to be honest with you. If anything I thought he over-achieved," Pivac told the BBC.
"I thought he played very, very well for his first game back. His decision making, the timing of the pass just to hold the defenders and put guys away - that's class.
"I guess it would have put a smile on Warren Gatland's face with the summer tour just around the corner."
Scarlets victory was part of the hugely-successful Judgement Day IV, with Ospreys also beating Cardiff Blues 40-27 at the Principality Stadium.
Williams lasted 52 minutes on his return and the 34-cap centre was delighted to be back in the thick of it.
"It was better than I thought," he said.
"It's great to be back - it's been a long time and I'm just happy to be back with the boys.
"But it was tiring and I thought I might not make it to half-time."
To buy tickets for the Guinness PRO12 Grand Final click here. Also follow us on Facebook, join the conversation on Twitter, sign up to our YouTube channel for extensive match highlights and sign up for our newsletter for regular updates on the Guinness PRO12.