Harrington, who has held the job since the third game of his rookie (2002) season - except for two games he missed for surgery to correct an irregular heart beat, is under fire for an offense that has been sluggish and unproductive.
Garcia, who was a three-time Pro Bowl quarterback for coach Steve Mariucci when they were in San Francisco in the 2000-2002 seasons, is coming off a broken fibula but is perceived by some as the savior of Mariucci's West Coast offense in Detroit.
They are splitting the reps in practice this week and Mariucci says he will make a decision "later on in the week."
The criterion for the decision?
Part of it will be Garcia's physical condition - whether his broken leg has healed sufficiently to allow him to play with the mobility and escapability that made him effective with the 49ers - but there will be other factors as well.
"We'll take that into consideration, how they practice, how they feel and then I've got to decide who gives us the best chance to win this game," Mariucci said. "We make most of our decisions - at any position - with that in mind: Who gives us the best chance to win this game?"
Although there was a perception at the start of training camp that Mariucci was chomping at the bit to get Garcia into the starting job, that feeling - if it ever existed - might have been tempered by Garcia's unimpressive play in the preseason and the fact he missed six weeks of practice with the broken leg.
There is little doubt Garcia understands Mariucci's offense better than Harrington, has more experience in that offense than Harrington and is probably better suited to the short, accurate throws required in the West Coast offense.
At 35, however, Garcia's modest arm strength has faded somewhat and the problems facing the Lions involve more than poor play by the quarterback.
Harrington, admittedly, has not played well. He has completed only 53.1 percent of his passes, has four touchdown passes to eight interceptions and his passer rating of 55.6 is the worst among NFL starting quarterbacks. But he has frequently been the victim of an offense that is seemingly incapable of handling opposing blitzes, holding off the normal pass rush or opening holes for the running game.
The three first-round receivers who were expected to bring offensive fireworks to the Lions have fizzled. At least so far. Charles Rogers is two games into a four-game suspension for violating the NFL's substance abuse policy, Roy Williams has been only so-so and missed the Carolina game with a quadriceps injury, and rookie Mike Williams is still learning his job.
The running game - expected to provide a needed balance for the passing attack - is 30th in the NFL with an average of 88.6 yards per game - and running back Kevin Jones, who had such an impressive second half of his rookie season a year ago, is averaging only 2.7 yards per carry.
Regardless of whom Mariucci selects as the starting quarterback Sunday at Cleveland, it's doubtful he will have solved the problems with the Lions offense.
SERIES HISTORY: The Lions and Browns will be meeting for the 17th time in
regular season competition, with the Lions holding a 12-4 edge. The teams also
met four times in NFL championship games in the 1950s, with the Lions winning
the title in three of them.
Joey Harrington isn't conceding an inch in the competition for the Lions quarterback job but - with the team struggling to score points - he might be fighting a losing battle this time.
Veteran Jeff Garcia was cleared to play this week after missing six weeks of practice and games with a broken left fibula and badly sprained left ankle, and is sharing reps with the first offense in practice this week.
Harrington, nicknamed Joey Blue Skies by some of his critics for his upbeat, positive approach, has been booed unmercifully by Ford Field fans and trashed from stem to stern on the sports talk radio station.
Yet he has remained remarkably resilient.
"They will not break me," Harrington told Detroit Free Press columnist Mitch Albom. I'm not gonna quit. They will have to drag me out of here. I know I'm still a good football player."
Even if Mariucci benches him for Garcia, Harrington says he will continue to prepare as if he would be the starter.
"I'm telling you," he said after sharing reps Wednesday with Garcia, "I don't care what happens, I'm absolutely committed to helping this team win and if it means that I've got to back up Jeff, then so be it.
"I'll be ready if - God forbid - anything happens ... I'm not going to quit. That is not going to be the end of me."