The former Heisman Trophy winner may play as much as three quarters as the Ravens want to evaluate him as they try to determine who will be the quarterback in the season opener against the Cincinnati Bengals on Sept. 7.
Former starter Kyle Boller is Smith's chief competition, yet he might not even play against the Rams. Boller has committed a team-high three turnovers in two preseason games.
Harbaugh emphasized that he hasn't made up his mind on who will ultimately be the starter, but this is obviously a prime opportunity for Smith to seize the job.
"He will play as much as we need to have him play," Harbaugh said. "The thinking is that Kyle has had 42 starts and we have a good idea of what he is and what he's been. We need to find out more about Troy Smith. He needs those reps. He needs them against a defense in a third preseason game where they're game-planning him a little bit and we'll see how he does."
Smith has completed just 8 of 17 passes for 99 yards and no touchdowns with one interception. He has a 41.1 quarterback rating, which is the lowest among the team's three quarterbacks.
Smith said he wasn't surprised to be the starter for the third preseason game. Traditionally, that's when the starters play the most before the regular season.
"No, I say that because I am confident in what I can do and what I will do," Smith said. "Obviously being a young player, I haven't had that much time on the field. I'm very, very thankful for the situation and the opportunity and I'm grateful for everything to come."
Smith threw just five passes when he started the Ravens' 23-15 loss against the Minnesota Vikings on Saturday, completing three for 25 yards with one interception. Accuracy has been an issue for Smith, who threw two touchdowns with no interceptions as a rookie last season in two starts.
"It will come," he said. "Anytime passes are in the game plan, you want to try to hit them all, but nothing is perfect when you get out there on the football field. This is still the preseason. We've still got a long way to go."