Mullens completed 16 of 22 passes for 262 yards and three touchdowns with no interceptions, as he stepped into the starting lineup in place of injured C.J. Beathard.
Mullens compiled a 151.9 passer rating -- the best performance in an NFL debut among players with 16 or more pass attempts in league history. Marcus Mariota had a perfect 158.3 rating in his 2015 debut for the Titans, but he had just 15 pass attempts.
Mullens is one of just three players since 1950 to throw for 250 or more yards, three or more TDs and no interceptions in his debut. The other two are Hall of Famers: Fran Tarkenton (1961) and Jim Kelly (1986). Kelly, however, was not making his first professional appearance. He previously played two seasons in the USFL.
Mullens became the first 49ers quarterback, in his NFL debut, to throw two or more touchdown passes. He also tied the franchise record of TD passes in his first start, joining Tim Rattay (2003), Ty Detmer (1998) and Matt Cavanaugh (1984).
But Mullens wasn't alone in his stellar debut. Here are some Pro Football Focus metrics from the 49ers’ most–lopsided win since a 42-10 victory over the Jaguars in 2013:
-- The 49ers’ offensive line of Joe Staley, Laken Tomlinson, Weston Richburg, Mike Person and McGlinchey gave up only one quarterback pressure. Person allowed one hurry.
San Francisco 49ers
-- Tight end George Kittle made four catches on his four targets for 108 yards. Each of his targets gained a first down or went for a touchdown. He leads the NFL among tight ends in receiving yards (692), receiving yards per route run (2.7) and yards after the catch (490).
-- Outside linebacker Dekoda Watson played just 13 pass-rush snaps but totaled six pressures with 1.5 sacks and four hurries. Watson made his first appearance of the season after spending the first eight games on injured reserve with a hamstring injury.
-- Defensive end Cassius Marsh played 22 pass-rush snaps and recorded 2.5 sacks and three quarterback hurries.
-- Cornerback Richard Sherman and Ahkello Witherspoon allowed just 57 receiving yards. Sherman allowed receptions both times he was targeted but for just 12 yards. Witherspoon gave up three catches on four targets for 45 yards.
Here are the total numbers of snaps of every 49ers player who saw action:
Total offensive snaps: 55
Quarterback -- Nick Mullens 55
Running back -- Kyle Juszczyk 34, Matt Breida 27, Raheem Mostert 14, Alfred Morris 10, Earl Mitchell 1
Wide receiver -- Marquise Goodwin 40, Pierre Garçon 37, Richie James 23, Kendrick Bourne 12, Dante Pettis 11
Tight end -- George Kittle 33, Garrett Celek 20, Ross Dwelley 13
Offensive line -- Laken Tomlinson 55, Mike McGlinchey 55, Mike Person 55, Joe Staley 45, Weston Richburg 45, Garry Gilliam 10, Joshua Garnett 10
Total defensive snaps: 57
Defensive line -- DeForest Buckner 40, Cassius Marsh 31, Arik Armstead 30, Solomon Thomas 30, Ronald Blair 22, Earl Mitchell 20, Sheldon Day 19, D.J. Jones 18
Linebacker -- Fred Warner 51, Malcolm Smith 33, Elijah Lee 25, Mark Nzeocha 19, Dekoda Watson 17
Defensive back -- Ahkello Witherspoon 57, Tyvis Powell 57, Jimmie Ward 51, Richard Sherman 47, K’Waun Williams 38, Tarvarius Moore 10, D.J. Reed 6, Marcell Harris 6
Total special teams plays: 27
Lee 20, Nzeocha 20, Moore 19, Bradley Pinion 17, Reed 15, Harris 15, Watson 14, Mostert 13, Dwelley 13, James 12, Kyle Nelson 11, Marsh 10, Ward 9, Buckner 9, Witherspoon 8, Jones 8, Robbie Gould 7, Tomlinson 7, McGlinchey 7, Gilliam 7, Garnett 7, Staley 7, Armstead 7, Powell 6, Celek 5, Bourne 4, Pettis 4, Emmanuel Moseley 3, Juszczyk 3, Williams 3, Thomas 3, Mitchell 2, Blair 1, Day 1.