Mullens should have tried to run for the first down rather than throw the football. The 49ers ended up losing the game 14-9.
With everyone on the same page, Shanahan told reporters via a conference call on Monday that he doesn't need to rehash the situation with his young quarterback.
"We'll watch it together so we can go over it," Shanahan said. "You don't have to rehash it very hard when you're dealing with a smart, honest dude who keeps it real and will admit it to you. Nick knows what he missed. By the time he came to the sidelines when I asked him, he already knew. Again, that's just part of playing the position, and it's tough.
"Nick made a lot of plays yesterday. A number of them, when he broke the pocket looking for a big play instead of looking to run. That's one of the reasons he hit Kittle on a big one going across the field. That's the one when he hit Kittle in the end zone, which I definitely think should have been [pass interference]. But it gave us a chance for a touchdown.
"And then he got out at the end, and he broke (the pocket). He was not really thinking about running. He had throwing in his mind, and he sees someone down the field. Obviously, he made the wrong decision, and he knew that before having to watch the tape. He knew that by the time he got to the sideline.
"You learn from it and hopefully next time, if he's in that situation, he'll run it. Also, if [Marquise Goodwin] is a little more open and he throws it and 'Quise catches that ball, and we win the game, I know we're all going to be pretty happy with him too. You've got to take some of the good with the bad. He knows that he missed that. We all know he missed it. Hopefully, next time in that situation, he'll make the right decision."
Mullens said after the game that he knew the decision was the incorrect one as soon as the football left his hands.
"If you look at the film or whatever, I mean I stood there for 30 seconds just understanding what I've just did, and how big of a mistake it was," Mullens said.
As Shanahan explained, Mullens was merely trying to make a play to help the 49ers win the game.
"You see Marquise down the field (and) I just wanted to give him a chance for a big play," he added.
Chalk it up to yet another learning experience for the second-year quarterback who hopes to turn things around during Sunday's finale against the Los Angeles Rams. Mullens, who is now 3-4 as a starter, can get back to .500 with an upset victory over the division rival.
The 49ers are treating this week like one following a Monday night game. Players have Monday off to be with their families on Christmas Eve, and they always have Tuesday off. The team will get back to work on Wednesday while the coaching staff uses Monday to work on its final game plan of the season.