WESTFIELD, Ind. — The Indianapolis Colts have wrapped up their first week of 2018 training camp practices at Grand Park Sports Campus in Westfield, Ind.

What are four takeaways from the first four practices?

1. Andrew Luck is quickly getting back to form

Andrew Luck admitted to having jitters prior to stepping out onto the Grand Park practice field for the very first time on Thursday. It's completely understandable, considering he was about to be a full participant in a team practice for the first time since December 2016.

Luck certainly didn't take long to start knocking the rust off in that first day of action, utilizing mostly shorter pass attempts. In all on Thursday, he completed 11-of-19 passes combined in 11-on-11 and 7-on-7 action with one interception.

But Luck really started to get some momentum behind him about midway through his second practice session on Friday, as he started going down the field more frequently, completing big gains to tight end Eric Ebron and wide receiver Deon Cain.

With a scheduled day off from throwing on Saturday, Luck returned to the practice field Sunday for his first fully-padded session of the year, and he really seemed to pick up where he left off on Friday. In all, Luck completed 19-of-22 passes in 11-on-11 and 7-on-7 work with two touchdowns on the day, and he also had a couple nice long completions to wide receiver T.Y. Hilton that were vintage Luck-to-Hilton looks.

Luck admits he still has a ways to go to get back to where he wants to be, but through three full practices he's shown obvious improvement and growth already. Head coach Frank Reich said he expects Luck to take on a normal workload in the preseason, which, outside of more practices will really be his best test prior to taking the field for the Colts' 2018 regular season opener against the Cincinnati Bengals.

2. The 2018 draft class has come to play

While several members of the Colts' 2018 NFL Draft class were either limited or did not participate at all during the offseason workout program, several picks have already impressed in just a week's time at training camp.

First-round pick Quenton Nelson has been as solid as advertised as the Colts' starting left guard. When the pads came on Saturday, however, Nelson really stood out, especially in 1-on-1 drills against opposing defensive tackles. Oh, and he's already gotten under the skin of a couple of his defensive teammates, showing some of that nasty streak we heard so much about.

We didn't quite know what to expect out of second-round pick Darius Leonard, meanwhile, considering he missed the entire on-field portion of the offseason program as he rehabbed an injured quad. But the linebacker was labeled full-go for the start of training camp, and it appears as though he hasn't missed a beat, picking off Luck with an athletic grab in coverage on tight end Erik Swoope on the very first day of practice, and then showing off his speed working in with the first-team defense ever since.

But perhaps the top rookie performer in camp so far is sixth-round pick Deon Cain. The wide receiver out of Clemson seemed to really be catching on as the offseason program came to a close, and he's certainly carried that momentum with him to Westfield. Cain has made at least one, if not more, impressive play during each practice, and his body control when going up against a defender has been a real treat to watch.

3. Early injuries up front

The Colts, on paper, have perhaps their deepest offensive line group in years — and certainly during the Andrew Luck Era (since 2012). But a few minor injuries have kept the big men up front from really forming any sort of cohesion for the first part of camp.

Starting left tackle Anthony Castonzo (hamstring), as well as Austin Howard, who could very well be considered the starter at right tackle (undisclosed injury), each started camp on the Active/Non-Football Injury (NFI) list, meaning they suffered injuries while away from the team during the break between veteran minicamp and the start of training camp. Guard Jeremy Vujnovich, who started all 16 games for the Colts last season, missed the entire offseason workout program with a calf injury and started camp on the Physically Unable to Perform (PUP) list.

Castonzo, Howard and Vujnovich can all be taken off the NFI and PUP lists at any time and return to practice, but that hasn't happened so far through the first week of camp, leaving the Colts a bit short-handed. Jack Mewhort, who is naturally a guard, has worked in with the first team at left guard in Castonzo's place; Denzelle Good has worked in at right tackle. Then, when Mewhort had a veteran's day off, the team put Tyreek Burwell at left tackle, but he suffered an injury early in Saturday's practice and is yet to return.

So that has led to the team utilizing second-round pick Braden Smith — again, more naturally a guard — at right tackle at times. Third-year veteran Le'Raven Clark has also gotten some first-team reps at left tackle.

The bad news is in an ideal world you want your starting five offensive linemen to use training camp to really begin to mesh together to be ready for the regular season. The good news, however, is that several remaining players — many of them rookies or on the younger side — are getting key practice reps at multiple positions.

Reich said he doesn't expect Castonzo or Howard to be on the sidelines for much longer; he just doesn't see the point in rushing them back at this point and risking further injury.

4. The depth chart still isn't clear

We can give an educated guess at the current Colts starters on both sides of the ball (not factoring injuries):

QB: Andrew Luck

RB: Marlon Mack

TE: Jack Doyle

TE: Eric Ebron

WR: T.Y. Hilton

WR: Ryan Grant/Chester Rogers

LT: Anthony Castonzo

LG: Quenton Nelson

C: Ryan Kelly

RG: Matt Slauson

RT: Austin Howard

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DE: Jabaal Sheard

DT: Al Woods

DT: Denico Autry

DE: John Simon

LB: Darius Leonard

LB: Anthony Walker

LB: Najee Goode

CB: Pierre Desir

CB: Kenny Moore II

S: Clayton Geathers

S: Malik Hooker

But some of these players — Castonzo, Howard, Geathers and Hooker — are yet to practice during camp as they continue rehabbing injuries, while others are listed based on what has been seen the first four days of camp.

Point being: these names certainly aren't etched in stone one bit.

While the offense is a little more certain, new Colts defensive coordinator Matt Eberflus has been cycling several players in at just about every position each day, making it difficult to get a read on who, exactly, would fill out the depth chart.

At defensive end, for example, we saw Tarell Basham getting starters' reps, but after a couple days of training camp practices, Simon has seemed to take over.

At cornerback, it seems as though Nate Hairston and Quincy Wilson are also "starting" candidates for the Colts, depending on whether the team opens up in the nickel package or not.

And at safety, with Geathers and Hooker out, it's been Matthias Farley and T.J. Green primarily filling in during the offseason and the first part of camp.

The point: who "starts" a game for the Colts this season might matter less than it ever has. Especially on defense, the team wants to come at the opposition in waves, so all kinds of combinations can and will be used. Stay tuned, though, as these roles should solidify at least a little bit more over the next few days.