When organized team activities start, and big men become athletic with the slightest of contact, injuries often can be the result.
On the very first day of OTAs for the Denver Broncos this year, offensive lineman Nico Falah suffered a season-ending Achilles tendon injury. No hitting, nothing really different than a normal NFL practice day and he's done for this year.
Last year on the first day of OTAs, San Diego tight end Henry Hunter blew out his right ACL.
Bears general manager Ryan Pace always tries to address team depth in the offseason, but stuff happens, like with Hunter and Falah.
If it happens to the wrong people, the season can become anywhere from a challenge to downright intolerable.
As the Bears get ready for Tuesday's start to OTAs, here are their indispensables. To varying degrees, they'd be replaced but in each case it would mean a drastic, damaging downgrade and possibly prevent much team success this season.
Not surprisingly, most of them are defensive. As the Bears become better on offense and score more points, they make defensive players more dispensable. It hasn't happened yet.
No. 6 Safety Eddie Jackson
He's an All-Pro and came into his own in season No. 2, and the Bears do have Ha Ha Clinton-Dix now at safety to accept some of the pressure if something happened to Jackson. However, Clinton-Dix is only in his first season within the system and Jackson has valuable system knowledge, even if it does change slightly under new defensive coordinator Chuck Pagano.
They played without Jackson's ability to shut down big plays when they faced the Eagles in the playoffs. While they managed to prevent most big plays, a few did occur and one question Bears fans will always ask is what could have happened if they'd only had Jackson, tight end Trey Burton and slot cornerback Bryce Callahan for the playoff game.
No. 5 Nose tackle Eddie Goldman
In the middle of the line, it's often easy to miss what Goldman accomplishes for the defense. They ranked No. 1 against the run and it was on that base where they were able to build a pass rush. The run defense always makes the pass rush possible.
Goldman had what Pro Football Focus called a "breakout season" in 2018 but if they'd watched close enough he actually did that in 2017. Goldman helps make it possible for Khalil Mack and Jackson to do what they do best, and occupies blockers so Danny Trevathan and Roquan Smith can snuff out the run.
What makes Goldman so indispensible is they actually have no real replacement. Any player they put at nose tackle would give up at least 15 pounds or more as well as the strength Goldman has. If they moved Akiem Hicks over to nose, it wouldn't be a loss of skills and size, but it would be weakening two positions.
No. 4 Cornerback Kyle Fuller
Fuller tied for the NFL lead in interceptions last season and now plays man to man and zone equally well. At one time he was considered more of a zone defender. He is the true Bears lockdown cornerback, and there is no one else on this level as his replacement. They could move Prince Amukamara to the defense's left side if need be, and then slide either Buster Skrine, Kevin Toliver or Sherrick McManis in at the right cornerback spot. Or they could just put Skrine on the left side of the defense in Fuller's place. Skrine actually played outside in Cleveland before switching to the slot with the Jets. Either option leaves a gaping hole.
No. 3 Defensive end Akiem Hicks
Hicks commands respect in the locker room and on the field. The defense started making its move up the league statistical standings when he came over from New England. There is no one with his ability to leverage 325 pounds against blockers or to shoot into the backfield and make a stop, or even get after the passer. He is to the Bears line what Aaron Donald is to the Rams. Without Hicks, they'd use Roy Robertson-Harris, Jonathan Bullard and Bilal Nichols a little more each. They're much lighter and not as strong.
Nor are they the leader Hicks is.
No. 2 Quarterback Mitchell Trubisky
Detractors like Pro Football Focus, Peter King and Sports Illustrated would have you believe otherwise, but the Bears could ill afford to lose Mitchell Trubisky for any length of time. As young player, inconsistency is present. However, his big plays far outnumered his turnovers and his quickness moving in the pocket or leaving it constantly allowed the Bears to avoid poor down and distance and continue drives.
And then there is the backup side of it. Chase Daniel knows their offense in and out based on years in Kansas City and Philadelphia, but he has only four NFL starts. Backups rarely succeed at quarterback. Last year backups went 27-46-1 when forced into starting roles, and of the 20 who made starts, only four had winning records.
Basically, with Trubisky it's like it is with almost every team that doesn't have Nick Foles as a backup. You lose. They were 1-1 last year against two bad teams with Daniel calling signals.
No. 1 Outside linebacker Khalil Mack
There is a reason Bears coaches call Mack the "multiplier." He made everyone so much better. They went from a stout defense, to one capable of being the aggressor. As a result, they forced turnovers in bunches, 27 interceptions after only eight in each of the three previous seasons.
The Bears saw what can happen when they are without Mack for a period of time, or when he is playing on one good leg. They went 2-2. The rest of the year they were 10-2.
Aaron Lynch is fine as a backup, but it's a huge drop when they have Lynch teamed with Leonard Floyd at outside linebacker as opposed to Mack with Floyd.
It's not one they'd care to experience again.