Getting an opportunity to play because Martavis Bryant was suspended for the year, Coates caught 20 passes for 425 yards and two touchdowns in the first five games of the season.
Coates had six receptions for 139 yards and two touchdowns in a Week 5 game against the New York Jets -- his best game as a professional -- but that day also marked the date his season began to unravel.
Coates dove for a ball in the second quarter and split his middle finger open. He had to get stitches at halftime.
From that point on, he had only two more catches for 14 yards over the final 11 games of the season as he struggled to fight through that injury, another fractured finger and a sports hernia.
"I started off so hot and then I wasn't the same player anymore," Coates said Tuesday as the Steelers started their second week of OTAs. "At the end of the day, you can't let that happen to yourself. It's all mental. You can't let it get in your head.
"Once you have one bad game it kind of becomes a domino effect. It goes to the next game. I started thinking more about my fingers instead of just playing football. I had to learn from that."
The Steelers didn't bank on Coates to get his act together this off-season. In addition to welcoming Bryant back after his year-long suspension, they drafted USC receiver Juju Smith-Schuster in the second round of the draft.
Clearly, they weren't happy with the way Coates finished the season.
Now Coates finds himself in a battle for a roster spot. Bryant, Smith-Schuster and All-Pro Antonio Brown are all but assured of roster spots.
Second-year slot receiver Eli Rogers is a pretty good bet, too. That leaves Coates in competition with veterans Darrius Heyward-Bey and Justin Hunter, as well as second-year receiver Demarcus Ayers for the fifth and sixth receiver spots.
"It's football," Coates said. "It's competition. You have a room like that, where a bunch of guys can go anywhere in the country and be a starter. ... There's a lot of competition. Competition makes us get better."
Steelers sign safety: The Steelers signed safety Daimion Stafford to add some depth at safety, where they are a bit thin. Stafford, who started six games for Tennessee last season, did not get much attention during free agency. His only other visit was to Jacksonville.
Stafford is expected to add depth behind starting strong safety Sean Davis and veteran Robert Golden, who started 11 games over the last two seasons as the primary backup at both safety spots.
"I thought I had a pretty good year last year," Stafford said. "I can control what I control. I did what I did on the field. If one team didn't love me, that's just how it was. The Steelers are giving me a chance, and I'm here to make the best of it."
The Steelers also signed long snapper Kameron Canaday and released punter A.J. Hughes.
Davis had surgery: Second-year safety Sean Davis revealed he had surgery for a torn labrum over the off-season. He's been on the field doing individual work but hasn't taken part in the team portion of practices.
Conner's return limited: Rookie running back James Conner returned to practice on a limited basis. He injured a hamstring during rookie minicamp and missed the first week of OTAs. With Conner injured and Le'Veon Bell absent while rehabbing from sports hernia surgery, it's left plenty of first-team reps for Knile Davis, a free agent who was signed in March.