The Indianapolis Colts began their on-field preparation for Sunday's road matchup against the Pittsburgh Steelers, and they were missing five key players, including four starters.

A positive development, however, was tight end Will Mallory returning to practice after spending the first four weeks sidelined on Injured Reserve with a thumb injury. The Colts have 21 days to activate Mallory from IR or keep him on it for the remainder of the season.

The Steelers don't have a long list of injuries, but the players who are on the injury report are notable.

Here is how both teams' health is shaping up ahead of Sunday's matchup.

Arguably the most significant injury to monitor for the Colts is Bortolini, the starting center. His communication throughout the week and on gamedays with quarterback Daniel Jones is vital to the Colts' offensive success, and if Bortolini misses a road matchup in a loud, rowdy environment, it could be a huge challenge for Jones and backup center Sedrick Van Pran-Granger.

Both of the Colts' starting linebackers, Davis-Gaither (MIKE) and Carlies (WILL), missed Wednesday's practice, although Davis-Gaither also missed some time last week and still played. With Davis-Gaither out, it gives rookie second-round pick CJ Allen more of an opportunity, as it does fellow rookie Bryce Boettcher with Carlies sidelined. Austin Ajiake also factors in on passing downs.

Key suffered a hamstring injury in last Sunday's win over the Washington Commanders and is expected to miss multiple weeks. Jaylahn Tuimoloau is likely to start in his place, with rookies Caden Curry and George Gumbs Jr. likely to see more action. The Colts also signed veteran edge defender Carl Lawson to the practice squad this week.

Dulin missed Weeks 2 and 3 with an ankle injury and came out of Week 4 with a hamstring. Without him in the lineup, expect to see more of Laquon Treadwell on offense and Anthony Gould on kickoff returns.

Allen missed last week's game with his injury but was able to return to practice in a limited capacity on Wednesday.

Dean and Ramsey make up two-thirds of Pittsburgh's starting cornerback group, so their status is critical to monitor.

Former Colt Pittman, who was traded to the Steelers this offseason, has been dealing with a foot injury that kept him out of Week 2, but he's played through the two games since.

The Colts and Steelers face off on Sunday at 1:00 p.m. ET on CBS in Pittsburgh at Acrisure Stadium.