He'd seen the acrobatic grabs and run-after-catch ability from the second-year pro.
But with the Los Angeles Chargers' top receiver Keenan Allen out with a hip injury, Rivers and the rest of his teammates needed to see it against the Kansas City Chiefs, with the Bolts down 14 points in the fourth quarter on Thursday.
And Williams came to his team's rescue.
"There's never been any doubt in this locker room about Mike," Rivers said. "But I think that the whole world knows now why he was picked No. 7 [overall in the 2017 draft], you know? ... All the receivers contributed, but he took over the football game."
Williams put on an impressive display, finishing with six receptions for 76 receiving yards and three total touchdowns. His effort included the final score on a 1-yard fade route, along with the two-point conversion to close out the 29-28 victory.
Williams also scored on a 3-yard touchdown catch in the second quarter and ran in a 19-yard reverse for a touchdown.
The Clemson product had a disappointing rookie season, missing six games last year due to back and knee injuries and finishing with just 11 receptions for 95 receiving yards.
With that experience behind him, Chargers left tackle Russell Okung said the start of Williams' heroics began during training camp with daily study sessions and drills with new receivers coach Phil McGeoghan.
"A lot of people can be excited about the plays he made today," Okung said. "But what I remember is training camp, and this guy coming back from an injury and trying to prove himself in the NFL.
"What he and Phil McGeoghan did, and the effort that me made every day to show that he can be an integral part of this team -- that process that he's been through and everything he put together -- all of that was evident today."
From there, Williams gained more trust in himself, with Rivers and his role in the offense. So when Allen went down Thursday, Williams knew it was time for him to step up.
"All the receivers told [Allen] that we were going to make the plays that needed to be made," Williams said. "Travis [Benjamin] made some big plays on that last drive, Tyrell [Williams] also made some big plays in the game.
"We just wanted to step up. [Allen] wanted to play in this game. He was down, so we just wanted to fight for him."
With two games left, Williams has totaled 37 receptions for 592 receiving yards and nine touchdown catches.
Along with Williams, Benjamin played much bigger than his diminutive, 5-foot-10, 175-pound frame.
Benjamin made two huge catches on the winning drive for the Chargers, a 31-yarder over the middle on which he was drilled by Kansas City safety Ron Parker and held on to the ball, and another 26-yard catch on fourth-and-7 that kept the final drive alive.
Benjamin was an honorary captain for the game and took over the role as slot receiver with Allen out.
"The way he has been practicing, it did not surprise us," Rivers said about Benjamin. "We have been seeing him practice. He started out the season pretty slow because of injuries, but once he's gotten healthy, he has looked very good. It was good to see him have some success on the football field tonight."
Williams was the latest hero for the Chargers as they have won or held on to win four games this season in the last seconds of the game.
Chargers head coach Anthony Lynn said he decided early on that his offense would go for two and the win if the Chargers got into the end zone on the final drive.
"We didn't come here to tie, we came here to win," Lynn said. "To me it was a no-brainer."
For Lynn and his players like Williams, they have embraced making plays in the tense, final moments of the game.
"We have been in these situations so many times, we believe we are going to win now," Lynn said. "The guys are really comfortable when we are in these situations. They do not panic. There is no anxiety."