INDIANAPOLIS – They remain two of the more underappreciated players on the NFL radar over the past 24 seasons.

That would be Tarik Glenn and Anthony Castonzo.

They offered the Indianapolis Colts stability at the most critical position on the offensive line – left tackle – and did so while playing at a high level. Glenn was a three-time Pro Bowl selection. Castonzo never caught the eye of enough voters despite adeptly fending off edge pass rushers, and, like his predecessor, doing so with little help. A left tackle capable of handling his business one-on-one allows for a fuller, more versatile offensive game plan.

In 20 of those 24 seasons, Glenn and Castonzo started 298 of a possible 320 regular-season games. That’s stability personified.

Glenn suddenly retired during the 2007 offseason, ushering in four years of Tony Ugoh and Charlie Johnson sharing the workload before the Colts got it right by investing the 22^nd overall pick in the 2011 draft on Castonzo.

Castonzo’s retirement last month once again leaves the franchise with a massive hole. Four starters return from one of the NFL’s best offensive lines, but the missing piece arguably is the most important piece.

It’s worth reminding everyone the Colts were 4-12 when Castonzo missed a game with an injury during his career, 4-13 including the playoff loss at Buffalo. Last season, the Castonzo-led pass protection allowed 10 sacks in his 12 starts and 11 in the four games he missed.

“Anthony was a pillar of this team for the last 10 years,’’ general manager Chris Ballard said. “He played one of the toughest positions in the game at an extremely high level for a long time.’’

Now, a successor is needed.

Here are the options:

INTERNAL SOLUTION

• Gone: Anthony Castonzo (retired).
• Going?: Le’Raven Clark (expiring contract; suffered season-ending Achilles injury in week 13 at Houston); Chaz Green (expiring contract).
• Here: Quenton Nelson, Braden Smith, Will Holden, Carter O’Donnell.
• Comment: Clearly, the internal options rest with Nelson and Smith. And let’s not kid ourselves, this is a decision that could impact the integrity of the o-line.

The overriding question that Ballard, coach Frank Reich and position coach Chris Strausser must answer: Is Nelson, a generation talent and three-time, first-team All-Pro at left guard, capable of sliding to his left and playing at a high enough level? Or: Can Smith, who has emerged as one of the league’s best young right tackles, flip sides and handle the relocation?

The Nelson issue is intriguing.

“If that’s what happens, I’m going to tell you right now he’s going to do a nice job,’’ Castonzo said. “He’s got all the skills to do it.’’

Nelson occasionally took practice reps at left tackle with the scout team last season, more on a whim than anything else.

“He’d come up to me and be like, ‘Look at my set. What do you think?’’’ Castonzo said. “I’m like, ‘You know what? Honestly, it’s pretty good. Surprisingly, it’s pretty good.’

“He knows what comes with it out there.’’

Nelson experienced that in a competitive situation against the Raiders in week 14 when Castonzo missed a handful of snaps.

“We knew if we put him out there he is the kind of player that would rise to the occasion,’’ Reich said. “Looking at the tape of the handful of snaps that he got in there, it looked good.’’

Reich later added moving Nelson permanently to left tackle was “a realistic option.’’

Again, it’s an option that absolutely must work.

Listen to Ballard: “You’ve got an All-Pro guard, probably the best at his position. Does it make your team better moving him out of that spot . . . and can the replacement level player you put in (at left guard), he’s not going to play at Quenton’s level.’’

It’s a tradeoff worth making if Nelson is very good and not just serviceable.

If there’s any doubt, leave him alone.

As for the possibility of moving Smith to the other side, it’s not as simple as it sounds.

“This is the way an o-line coach put it to me: ‘It’s like playing golf when you are a right-handed golfer and you switch him to left-handed,’’’ Ballard said on the Dan Dakich Show on 1070-The Fan. “Eric Fisher played right tackle our first year in Kansas City, and then we kicked him over to the left side. It can be done. It can absolutely be done.

“But who are we putting in at right tackle? Those are questions we have to answer.’’

Moving either Nelson or Smith impacts two positions and would threaten the existing chemistry.

“We’ll let it all shift out through the offseason, and when we get to training camp play with some different combinations to make sure we get the best five on the field,’’ Ballard said.

VETERAN SOLUTION

• Trade options: Orlando Brown, Baltimore.
• Free agents-to-be: Trent Williams, San Francisco; Russell Okung, Carolina; Alejandro Villanueva, Pittsburgh; Jason Peters, Philadelphia; Cam Robinson, Jacksonville; Kelvin Beachum, Arizona.
• Comment: We’re not opposed to adding Brown or Williams, but the cost of either would be huge.

Brown, 24, has started 46 of 52 games and been named to two Pro Bowls since being selected in the third round of the 2018 draft by the Ravens. He started his first 35 games at right tackle before stepping in at left tackle for the final 11 games last season for the injured Ronnie Stanley. With Stanley’s return, Brown reportedly has asked Baltimore to deal him; he sees his future at left tackle. If the Ravens decide to deal him, they’ll demand a hefty trade package. And the team that acquires him faces the prospect of signing Brown to an extension worth more than $20 million per year.

Williams turns 33 in July but is viewed as one of the most attractive free agents-to-be, regardless of position. The 6-5, 320-pounder has started 135 of 136 games since being selected 4^th overall in the 2010 draft by Washington. He relocated to San Francisco last season, started 14 games and earned his eighth consecutive Pro Bowl appearance. Again, the cost would be high. Spotrac.com projects Williams getting a three-year free-agent deal worth $54.8 million ($18.3 million per season).

Villanueva, 32, has started 97 straight games for the Steelers and been to two Pro Bowls. Okung, 32, also is a two-time Pro Bowler who has started 145 games in 11 seasons with four teams. But the 6^th overall pick in 2010 (by Seattle) has missed 19 games the past two seasons with injuries.

DRAFT SOLUTION

• Top prospects: Penei Sewell, Oregon; Rashawn Slater, Northwestern; Christian Darrisaw, Virginia Tech; Alijah Vera-Tucker, USC; Jalen Mayfield, Michigan; Dillon Radunz, North Dakota State; Teven Jenkins, Oklahoma State; James Hudson, Cincinnati; Liam Eichenberg, Notre Dame; Samuel Cosmi, Texas; Walker Little, Stanford.
• Ammunition: 21^st overall pick in round 1; 22^nd pick in round 2, 54^th overall.
• Comment: Ideally, the draft once again delivers a top-end left tackle to the roster. The Colts sit 21^st overall, and that’s the range that produced their last two studs: Glenn 19^th in 1997 and Castonzo 22^nd in ’11.

By all accounts this is a deep draft at the position. Pro Football Focus’ top 100 prospects includes 12 tackles, led by Sewell (4^th overall), Slater (12^th) and Darrisaw (16^th). The Athletic’s Dane Brugler’s early mock draft has nine tackles selected in the first two rounds, including six in round 1: Sewell, Slater, Darrisaw, Vera-Tucker, Jenkins and Mayfield.

If the Colts opt to fill Castonzo’s void through the draft, it needs to be with a plug-and-play guy. There’s no one on the roster capable of starting for the first month while the rookie finds his comfort zone.

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You can follow Mike Chappell on Twitter at @mchappell51.