SANTA CLARA -- Safety Eric Reid recalls his Southeastern Conference matchups with running back Trent Richardson. Reid's memory, perhaps selective, is of stopping Richardson whenever Alabama's marquee tailback charged through LSU's defense.

Such a scenario could repeat Sunday at Candlestick Park.

Reid, the 49ers' rookie free safety, is on track to return from the concussion he suffered in Seattle on Sunday night. And Richardson, abruptly traded from the Cleveland Browns on Wednesday, will be making his debut for the Indianapolis Colts.

"I don't necessarily think we necessarily change our game plan," Reid said. "It just might take a little more oomph to take him down."

Despite having prolific quarterback Andrew Luck, the Colts indeed could unleash a strong dose of Richardson, who otherwise would platoon with Ahmad Bradshaw and Donald Brown.

"We did not bring him in here to be the water boy on Sunday, so he'll be ready to roll," Colts coach Chuck Pagano said at his Thursday news conference.

Vic Fangio, the 49ers defensive coordinator, indeed expects a heavy workload for the Colts' "upgrade" at running back. Richardson, said Fangio, comes with speed, strength, a sturdy stiff-arm and a shiftiness to run either inside or outside the tackles.

"It wouldn't surprise me to see him play half the game or 90 percent of the game," said Fangio, noting that Richardson should make a "seamless transition" from a Browns running scheme that is similar to the one the Colts run.

What would surprise the 49ers defense is if the Colts become a run-oriented offense. Instead, Richardson's addition should balance out an attack that saw Luck set rookie passing records last season for yards and attempts.

Linebacker Patrick Willis questioned why the Browns would move Richardson, whom he described as a "tough, downhill, physical running back" that requires defenses to swarm when tackling.

"This guy is a rolling ball of butcher knives," Pagano said of Richardson. "He fits our system, our scheme to a tee. ... It's great. You have a great young quarterback, a great young runner.

"We're trying to build a monster. We're trying to build a program for sustained success for the long haul."

The 49ers defense has done just that, succeeding in recent seasons behind a bevy of Pro Bowlers. Despite Sunday's loss in Seattle, Fangio was pleased with how his unit performed, with one notable exception.

"We didn't execute in the third quarter, not just in the secondary, but all over," Fangio said after the 49ers trailed just 5-0 at halftime.

Missing in that third quarter was Reid, who exited with a concussion after his second-quarter tackle of Sidney Rice.

"I had no headache, felt perfectly normal and wanted to go back and play, but they hid my helmet somewhere," said Reid, who must still pass a contact test before being cleared to play.

The Colts are also coming off a loss -- 24-20 to Miami -- and Wednesday's trade showed their urgency to rebound in their road opener.

"We have no choice but to attack it like that. There's no siestas in the NFL. You can't hug it out today," Colts general manager Ryan Grigson told reporters. "Yesterday was fun, but it's back to work. We have San Fran this week."

For more on the 49ers, see Cam Inman's Hot Read blog at blogs.mercurynews.com/49ers.

sunday's game

Indianapolis (1-1) at 49ers (1-1), 1:25 p.m. CBS

inside

Colin Kaepernick responding stoically after his worst passing game as a pro. PAGE 6