Grant Cohn: Tom Brady is the greatest … if you don't count Joe Montana

I can prove Joe Montana still is the greatest quarterback of all time.

But before I do, you should know The Press Democrat's Phil Barber recently wrote a strong, compelling column arguing why Tom Brady is the greatest quarterback of all time. I recommend you read it. He compared the two quarterbacks based on physical ability, stats, supporting cast and competition.

I will use my own criteria to make my case for Montana.

The way I see it, you can't even begin this debate until you account for the different eras Montana and Brady played in. Because, even though Montana and Brady both are quarterbacks, they may as well have played different sports.

In Montana's first season as a starter, 1981, the NFL's average quarterback rating was just 70.5. Passing was difficult. In 2017, the average NFL quarterback rating was a whopping 85.1. Passing was relatively easy.

So much changed since Montana played.

The NFL changed their rules to protect quarterbacks and wide receivers. These rules inflated passing statistics.

NFL offenses evolved from tight formations to spread formations, which create isolated one-on-one matchups. These make the quarterback's pre-snap decision-making process easier. He just picks the one-on-one matchup he likes best and throws in that direction.

Modern NFL quarterbacks are more prepared on a weekly basis than in Montana's era. They have more film and more detailed scouting reports.

Modern NFL quarterbacks have more opportunities to throw. They throw almost 40 times per game. In the past, they would throw about 25 times per game.

Modern NFL quarterbacks have more experience before they get to the league. Kids these days throw for thousands of yards in high school. In Montana's era, kids ran the Option and the Wing T Offense in high school. They didn't grow up taking five-step drops and reading coverages.

The evolution of the sport and the rules have given quarterbacks a huge advantage, which Brady benefits from.

To determine who is better, Brady or Montana, you have to address that advantage and find a way to compare the quarterbacks on equal terms.

Here's how I do it:

I put each quarterback's stats in the context of when he played. For example, Montana's quarterback rating in 1989 was 112.4. That season, the average quarterback rating in the NFL was 73.3. So, Montana's rating was 53.3 percent better than the average quarterback that season. Unreal.

Brady's quarterback rating in 2007 was 117.2, which was 44.9 percent better than the average NFL passer rating that season. Great, but not as great as Montana in '89.

Montana had nine seasons during his 15-year career when he posted a passer rating that was at least 20 percent better than the league average that year.

Brady has had only five seasons during his 18-year career when he has posted a passer rating that was at least 20 percent better than the league average that year.

Montana's 92.3 career passer rating was 27.7 percent better than the average NFL quarterback rating during the years he played — 1979 to 1994.

Brady's 97.6 career passer rating is 19.5 percent better than the average NFL quarterback rating during the years he has played — 2000 to 2017.

Montana had better regular-season numbers than Brady.

But, their legacies ultimately depend on their performance in the playoffs. And Brady's teams won more Super Bowls than Montana's teams. Brady's won five. Montana's won four. You hear that all the time.

Those are team accomplishments. I'm interested in which quarterback actually played better in the playoffs.

Montana's career postseason passer rating was 95.6. That was 32.3 percent better than the average rating in the NFL when he played.

Brady's career postseason passer rating is 90.9. That's only 11.3 percent better than the average rating in the NFL since he has played.

Not even close to Montana's level.

The only factor in Brady's favor is longevity. He stayed healthy. Montana broke down.

Longevity is important. Athletes get credit for protecting themselves and performing at a high level for an extended period.

But, Montana almost certainly would have lasted longer had he played in Brady's era. Defenders barely can breathe on quarterbacks now without committing a 15-yard penalty. Montana would have taken fewer big hits.

And his numbers would have been better. Much better. Just look at the effect the rule changes had on Brady's numbers.

From 2001 to 2003 – Brady's first three seasons as a starter — he posted passer ratings of 86.5, 85.7 and 85.9. Nothing special.

Then, in 2004, the NFL made a point of emphasis to enforce the 5-yard bump-and-run rule more strictly – defenders could not touch receivers after five yards. As a result of this one change, Brady's passer rating jumped to 92.6.

The rest of the league's passing stats exploded, too. From 2003 to 2004, the league's average passer rating rose from 76.6 to 80.9 — an increase of 4.3 passer-rating points. That's huge for one year. To compare, the league's average passer rating increased by just 3.5 from 1983 to 2003 — 21 years.

In 2008, Brady got hit in the knee and missed the entire season. Then, the NFL changed its rules again. It outlawed defensive players from touching quarterbacks below the knee or above the neck.

Since the NFL put that rule in place, Brady has missed zero games due to injury and his quarterback rating is 100.9.

Brady is great, don't get me wrong. But, he also is a product of his era.

Montana was ahead of his time. He was a prodigy. No quarterback ever will dominate the NFL like he did.

Grant Cohn covers the 49ers for The Santa Rosa Press Democrat and Pressdemocrat.com. You can reach him at grantcohn@gmail.com.