KANSAS CITY, Mo. -- Chiefs quarterback Alex Smith has a lacerated spleen and will not play in Sunday's game against the San Diego Chargers at Arrowhead Stadium. Backup Chase Daniel will start in his place.

Smith suffered the injury in last week's game against the Pittsburgh Steelers. He finished the game and even practiced two days this week before the injury was diagnosed Thursday.

Chiefs trainer Rick Burkholder said Smith will heal on his own and would not need surgery. He said a normal recovery time from such an injury is six weeks.

The Chargers will earn a playoff berth with a victory Sunday. The Chiefs could make the playoffs if they win Sunday but would also need the Baltimore Ravens to lose to the Cleveland Browns and the Houston Texans to lose or tie against the Jacksonville Jaguars.

Burkholder said Smith could return if the Chiefs advance deep into the playoffs.

"Generally [lacerated spleens] take six weeks [to heal], but everybody is different,'' Burkholder said. "He's a young, healthy guy that's got a high metabolism. If we're playing three, four weeks from now, we'll rescan him and see where he's at.

"From a long-term standpoint and from a health standpoint from Alex, he's going to be fine. That's going to heal up.''

Smith said he didn't notice the first symptoms until he was in the locker room after the Pittsburgh game. He said he felt symptoms he described as stomach discomfort, pressure and cramping.

"I thought it was something I ate,'' Smith said. "I didn't even necessarily relate it to a hit.''

Smith was examined afterward by team doctors, but no diagnosis was made.

"The reason Alex didn't have any symptoms Sunday that tipped us off is that he didn't bleed very much,'' Burkholder said.

Smith returned to Kansas City with the Chiefs on Sunday night. He worked out at the team's practice facility Monday, then practiced with the team Tuesday and Wednesday.

Smith said he wasn't at risk while practicing. Quarterbacks wear yellow jerseys indicating to the other players that contact should be avoided.

"I felt like I had two really good days of practice,'' Smith said. "It's obviously a strange thing to be dealing with. Both days I felt great athletically running around and throwing. I just kind of had this constant deal going on.

"It certainly wasn't something that was limiting me athletically. I just kind of had this discomfort and nausea kind of coming and going. There was a bug going around. I thought it was that.''

Smith said he even took an antacid at one point to ease his stomach.

Christmas Day was an off day for the Chiefs, but Smith finally went for a scan of his abdomen at the urging of Mike Monaco, the team physician.

"I was close to not going yesterday and just hanging with my family,'' Smith said.

Daniel started in Week 17 last season, too Chase Daniel has one career start -- Week 17 last season against San Diego. The Chiefs lost the game, but Daniel played well. A look at his performance: Total QBR 82.9 Comp-Att. 21-30 Pass yards 200 TD-INT 1-0 Rush yards 59 -- ESPN Stats & Information

The scan results showed a 3-centimeter laceration and a slightly swollen spleen.

"We'll continue to try to decide why his spleen is a little enlarged,'' Burkholder said. "It might just be ... from the trauma. But we're doing a little more [study] to cover ourselves that way and cover him and make sure we're on point.

"If he would take another shot and that laceration would increase, then you have a medical emergency.''

Smith said he planned to be on the sideline for Sunday's game.

"I didn't know what to make of it when they told me,'' Smith said. "I don't know all the medical stuff that comes with the spleen. You're sort of learning as they tell you. Once you start hearing how serious it is, you start knowing the implications that can come of this.''

Smith indicated his biggest regret was taking snaps for two days of practice that otherwise would have gone to Daniel.

"I really feel like I ate up two practice days of his that would have been important,'' he said.

Daniel, though, said Friday he is ready.

"I wouldn't rather have it any other way than to be put into a situation with everything on the line," he said. "I know the team believes in me and we'll go out there and try to put drives together against a good San Diego defense."

Chargers coach Mike McCoy said that although Smith is out, the Chiefs' "system is in place." The Chargers are facing Daniel as the Chiefs' starting quarterback in Week 17 for the second straight season.

"Andy's [Reid] has established a system there. And having faced him last year at this time, we've got to understand what type of player he is, and the success he's had in his career," he said "So we've still got to go out and play our best game."

Chargers linebacker Manti Te'o said he doesn't expect the Chiefs' offense to take too much of a hit with Smith out.

"The offense doesn't slow down at all with [Daniel] at the helm," he said. "It's basically the same thing, just a different number."

ESPN.com Chargers reporter Eric D. Williams contributed to this report.