After missing the first two games of the season while serving a suspension for violating the league's policies on personal conduct and substance abuse, Foster will make his season debut Sunday at the undefeated Kansas City Chiefs.
The 49ers could use Foster against the high-powered Chiefs, who have been an offensive machine behind quarterback Patrick Mahomes, but the second-year linebacker is just giddy to be back on the field.
"It's like a holiday, just knowing I can come back and play the game I love," said Foster, per NBC Sports Bay Area. "So I'm just happy to be playing the cards right and doing good off the field and on the field, and doing nothing off the field so I won't be off the field again."
Foster ranked second on the 49ers in tackles with 72 despite missing five games because of a high ankle sprain and another game with back spasms in 2017. He was named the NFL's Defensive Rookie of the Month in November.
"This is his first game back. We're excited to have him," said 49ers cornerback Richard Sherman. "He's excited to be back out there. He brings incredible energy and speed and explosiveness and physicality to the defense, and we appreciate it."
The Butkus Award winner at Alabama as the nation's top linebacker, Foster slid to the bottom of the first round as the No. 31 pick in the 2017 NFL Draft because of the character and medical issues (shoulder surgery after his final season with the Crimson Tide).
Kansas City head coach Andy Reid knows what Reid brings to the San Francisco defense.
"Reuben's a good football player," said Reid. "We looked at him when he came out, liked him, had him visit up here, the whole deal. He's a heck of a football player, and he flies around and makes plays. We're going to be aware of where he's at, and do our best to do well against him."