Mullens -- who had never thrown a regular-season pass entering the 49ers' win on Thursday against the Oakland Raiders -- completed 16-of-22 passes for 262 yards, three scores and no interceptions in the victory.
The Southern Mississippi product was given the start due to C.J. Beathard's wrist injury. Beathard was forced into action in Week 3 when 49ers starter Jimmy Garoppolo was lost for the season due to a torn ACL.
Shanahan said leading up to the game that Beathard would have played had the game been played on Sunday instead of Thursday. After the game, he was asked which quarterback would start in Week 10 against the New York Giants.
"Yeah, definitely we'll consider [Mullens]," Shanahan told reporters. "Definitely not thinking about that right now, but our whole team played very well. Nick definitely did. I thought our guys upfront played well. I thought the entire offense did. I thought the defense played their tail off. I thought the pass rush was there. I thought we tackled well. I thought our punter and kicker did well. It was a very good all-around win. I was real happy for our guys. We've been through a lot here and we're not done. We're trying to get our way out of this. It's good to get a win today and hopefully it can lead to some others."
Shanahan said the 49ers weren't surprised by the game not being "too big" for Mullens.
"He's very poised. He is always locked in," Shanahan said. "The guys, they respect him. He earned a lot of respect in the preseason, them watching him play. Both years, last year and this year. The way he handles himself in practice each week. He didn't really know for sure whether he was going to start today, until today, and just the way he handled the two days of long walk-troughs and stuff. He's a machine in there and I was happy for him."
Mullens might not have been well-known before getting the start, but during the game he saw his social media following rise and was verified on Twitter. He also got a phone call from NFL Hall of Famer Brett Favre after the game.
"He just told me how proud of me he was," Mullens told reporters. "He sent me a message before the game. He said, 'No pressure. Just be yourself,' and that's what I tried to do before the game and during the game. So, just tried to stay calm out there and lead the offense. Obviously, we got some veterans on the O-Line, so it was just a very collective effort."
The 49ers host the Giants at 8:15 p.m. on Monday, Nov. 12 in Santa Clara.