Johnson tore his Achilles tendon during Wednesday's practice, according to the team. The big receiver (6-foot-6, 220 pounds) out of Texas had been receiving first-team reps in recent weeks at practice with Kadarius Toney and Shepard among those on the sideline.
Coach Brian Daboll had noted earlier in the week that Johnson and fellow wide receiver David Sills were making a strong impression. Johnson started Sunday's preseason win over the Cincinnati Bengals and had three receptions for 41 yards; Sills had five catches for 56 yards, with most of the damage coming with the first-string offense.
"Collin Johnson, David Sills, they've stepped their game up," Daboll said. "And they're right in the mix, not just to make a team but to play."
Shepard, meanwhile, was making his way back from a torn Achilles suffered in December. He returned to practice for the first time Wednesday since the injury.
"It felt great, man," said Shepard, who snuck in some reps during live drills and caught a 20-yard pass from backup quarterback Tyrod Taylor. "Just being back on the field, it's a blessing. Especially when you go through an injury like that and you've been on the sidelines for so long watching those guys get after it. You have to take every day and cherish them because you never know when you're going to have to sit out that long."
It had been eight months and five days since Shepard tore his Achilles. He is the longest-tenured Giant and has 349 catches for 3,884 yards and 21 touchdowns in his career, all in New York.
Shepard's return made it appear as if the Giants were actually getting healthier at the position. Toney, Board and Slayton participated in the early portions of Wednesday's practice.
But Toney was seen grabbing at the back of his leg during the workout, and Johnson and Marcus Kemp (hamstring) suffered significant injuries. Both Johnson and Kemp landed on injured reserve later in the day.
Johnson, 24, has 29 catches for 377 yards in 26 career games with the Jacksonville Jaguars and Giants. He made his mark with the Giants last year as a blocker.
But this summer it appeared he had taken his game to the next level. He was making plays consistently throughout training camp, earning the trust of starting quarterback Daniel Jones and some first-team reps.
Johnson had 10 catches for 123 receiving yards in the first two preseason games this season. With Toney still banged up and Shepard coming back from a serious injury, it looked possible that Johnson could have started -- or at least played a substantial role -- Week 1 against the Titans. Instead his season came to a premature end.