The San Francisco 49ers' defense has been outstanding this season, and defensive lineman Arik Armstead is a big reason why.

Billed as an elite run-stopped heading into 2019, Armstead has also turned into a standout pass rusher, racking up 54 tackles, 10 sacks, a couple of forced fumbles and a fumble recovery this season.

Armstead is set to be a free agent this offseason, and while the 49ers may very well franchise tag him, there will be a whole lot of interest in him if San Francisco actually lets him walk.

One team that should absolutely pursue Armstead in that instance is the Indianapolis Colts.

The Colts aren't a terrible defensive unit, but they aren't great, either. While they were very good at stuffing the run (seventh in the NFL), they were mediocre to below average everywhere else.

The primary reason why Indianapolis is not a great defensive ballclub is because it doesn't have a ton of talent on that side of the ball, particularly along its defensive line.

Yes, there are names like Darius Leonard and Malik Hooker in the linebacking corps and the secondary, but outside of Justin Houston, the Colts' front four is lacking.

Armstead would quickly solve that issue.

Imagine Houston and Armstead rushing the passer up front? And imagine how much better Indy's already stout run defense would become if it adds Armstead, who is widely considered one of the NFL's best run-stuffers?

Of course, the Colts' biggest issue in 2019 was their injury-riddled offense, as just about every single one of their top offensive players was sidelined with injuries at some point this season.

But assuming Indianapolis can stay moderately healthy in 2020, its lackluster defense will still pose a problem, and it's something that Indy needs to address this offseason.

The Colts should also have a ton of cap room to work with, so they can absorb a big contract with no problem. Now, Indianapolis will still have to be cautious, as teams have deeply regretted handing out massive deals to defensive tackles in the past (e.g. Albert Haynesworth and Ndamukong Suh), but Chris Ballard and Co. also cannot deny the club's needs.

It should be noted that prior to 2019, Armstead had totaled just nine sacks over the course of his first four seasons, so it is a bit suspicious that he busted out like this in his contract year.

That being said, Armstead still displayed terrific talent as a run-stopper during that time, so perhaps he has honed his overall skills enough to the point where he is now a great pass-rusher, too.

Obviously, all of the talent along San Francisco's defensive line has surely helped Armstead, as well, but at 6-foot-8 and 280 pounds with freakish athleticism, you can see why Armstead could very well be developing into an elite defensive lineman.

Indianapolis has quite a bit to look forward to next season, namely a healthier offense. Remember: the Colts started 5-2 this year before injuries began taking their toll.

But Indy will be even more interesting if it adds some big pieces to its defense, and Armstead could be one of those guys.