It became inevitable the two would share a football field again when Harbaugh left to coach the San Francisco 49ers before Luck's junior season, but neither of them are focused on anything except beating the other as Luck's Indianapolis Colts - possibly with newly acquired running back Trent Richardson - visit Candlestick Park on Sunday.Indianapolis made a blockbuster deal Wednesday, trading a 2014 first-round pick to Cleveland for Richardson, the No. 3 overall selection in 2012 who rushed for 950 yards and nine touchdowns as a rookie.Harbaugh coached Stanford from 2007-10, helping build Luck into one of the most heralded college quarterback prospects over his final two years. After they led the Cardinal to an Orange Bowl victory, Harbaugh took the San Francisco job while Luck guided the Cardinal to a second straight BCS appearance before becoming the No. 1 overall pick by Indianapolis in the 2012 draft.Now, Harbaugh must help build a gameplan against his former pupil for the first time."He's a terrific player," said Harbaugh, who quarterbacked the Colts from 1994-97. "There's not a lot of weaknesses that he has. We're going to have to be very sound. He's a top-notch quarterback in the NFL."Luck, who set multiple rookie records last season, attributes part of his success to Harbaugh's tutelage."I definitely feel he helped me get to where I am," Luck said on a media conference call Wednesday. "He obviously helped turn Stanford around, his efforts more than anybody. Some of my success was being able to play at a well-coached, well-run program."They'll set sentimentality aside Sunday when the 49ers (1-1) and Colts (1-1) look to bounce back from losses.San Francisco beat Green Bay 34-28 in the season opener as Colin Kaepernick threw for a career-best 412 yards and three touchdowns, but he had 127 yards passing - his fewest in 12 career starts, including the postseason - and four turnovers in last week's 29-3 loss to Seattle.The 49ers had five turnovers overall, the first time they finished with more giveaways than points since committing five turnovers in a 28-3 loss to Indianapolis on Oct. 9, 2005."There's things that we'll put our finger on and try to make improvement, significant improvement, this week," Harbaugh said. "You want to identify where you can get better and make improvements, and then you want to see those improvements made."Harbaugh defended Kaepernick, whom Luck recommended to Harbaugh prior to the 2011 draft after the two met at Peyton Manning's passing clinic. The 49ers selected Kaepernick in the second round out of Nevada."Colin played hard and played well at times," Harbaugh said. "I'm not disappointed in the way Colin played at all. He did what he could."Luck also wasn't at his best last week in a 24-20 home loss to Miami, going 25 of 43 for 321 yards with a touchdown and an interception. He's been on the receiving end of seven sacks after being dropped 41 times last season, tying the most times a Colts quarterback had been sacked since Harbaugh in 1997."You've really got to limit your mistakes, and often times whoever wins comes down to who makes less mistakes," Luck said. "Or sometimes it comes back and bites you in the rear."T.Y. Hilton caught six passes for a career-best 124 yards, and Ahmad Bradshaw rushed for 65 and a touchdown in place of Vick Ballard, who is out for the season with a torn ACL."Everybody's miserable until you get to tee it up again the next Sunday," coach Chuck Pagano said. "(The 49ers) are coming off a tough loss, we're coming off a tough loss, so I think it will be the same in both places."The acquisition of Richardson may help the Colts feel a bit better, though. With Ballard out and uncertainty surrounding Bradshaw's ability to be a feature back, Indianapolis decided to bring in a player once considered to be Cleveland's running back of the future.Richardson, who rushed 31 times for 105 yards in the Browns' first two games, is expected to see at least some playing time Sunday as he gets up to speed.One of the biggest concerns for Luck and the Colts figured to be 49ers linebacker Aldon Smith, who already has 3 1/2 sacks after notching 16 last season, but Smith found himself in trouble with the law again Friday. He was arrested on suspicion of driving under the influence in San Jose at 7 a.m. - his second such arrest since 2012 - and also was in possession of marijuana.It's uncertain if Smith, who also has been named in a lawsuit by a Northern California man who said he was shot during a party at Smith's house last year, will play Sunday.While all eyes at the outset may be on the Harbaugh-Luck reunion, it won't be the only one Sunday. On Tuesday, the 49ers signed fullback Owen Marecic, who played with Luck and under Harbaugh at Stanford.