The Niners were up 3 with less than 10 minutes to go in the game ... when Rams QB Matt Stafford threw a pass directly to Tartt, hitting him square in the hands. Unfortunately for San Fran, Jaquiski dropped the should-be pick.

The Rams went on to kick a field goal, tying the game. Next possession, they took the lead for good, and punched a ticket to the Super Bowl.

Needless to say, the pick could've changed the outcome of the game.

And, Tartt knows it, and isn't making any excuses.

"No excuses!! I deserve all the criticism my way!" the 29-year-old said on social media.

"Opportunity I dream of I came up short!!! Let my brothers down!! It Still won't define me as a person/player. Only will get stronger and better."

Reporters asked Tartt about his post-game tweet, and he got emotional saying it should have been a game-changing play.

"I know I can make that play and when the play came up, I ain't make it," Tartt said.

"It hit my hands, I thought I had it and then I just ... I don't know how I dropped it. I didn't drop a ball in practice all week."

Tartt added, "I know that was a big play of the game, a big opportunity for me and for the team and as a player, I just feel like I let my brothers down."

Even though Tartt's placing the blame squarely on his shoulders, his teammates have his back.

49ers safety Jimmy Ward came to JT's defense over the dropped INT, saying no one play was responsible for the team's loss.

"There's a lot of plays out there, and we could have did better including myself," Ward said during a post-game interview.