It was about the most depressing rendition of "Happy Birthday" from a group of 89 guys you'll ever hear.

The Carolina Panthers ended Tuesday's minicamp practice singing to guard Trai Turner, who turned 23, and first-round pick Vernon Butler, who's now 22. But the song wasn't much of a celebration because of what it followed.

A fiery coach Ron Rivera gave his team an earful in a speech that included multiple reminders about a season opener in Denver that's just 86 days away.

When asked where the frustration came from, the usually even-tempered Rivera joked:

"I was just telling the guys how happy I was to have them here. I'm real pleased that they're all here and accounted for."

Except for receiver Devin Funchess, who was home sick, everyone was on the field for the mandatory session. Even defensive tackle Kawann Short, whose contract situation is now real sticky.

Despite the near-perfect attendance, coaches made it clear throughout the morning that this was far from a perfect practice. Frustrating moments included drops by receivers in team drills, bites on double moves by cornerbacks and another jump by the defensive line on a Cam Newton hardcount.

"We've got to go harder," safety Tre Boston said. "I expect more. We should all expect more from guys who went to the Super Bowl and didn't finish it.

"You can't just show up to practice every day and just lollygag around and think you're going to make it to the Super Bowl. We worked hard for it last year, and we've got to remember that."

But Rivera's main issue wasn't with the players who were in that Super Bowl.

"We've got a group of young guys that are going to get opportunities on this football field, and they've been doing these things for several weeks," Rivera said. "To see the mistakes that were made on very basic things really disappoints me more than anything else.

"You'll find that the guys that are really working at it, really doing the things that we need to won't make the mistakes that are being made right now."

The Panthers' draft class isn't big, but many of their roles could be. James Bradberry and Daryl Worley again lined up as the first-team cornerbacks with Zack Sanchez getting snaps behind them. Butler is part of the top rotation at defensive tackle while seventh-round pick Beau Sandland made the catch of the day, a one-handed snag near the pylon.

But one pretty play didn't disguise the bigger picture. Three months isn't a long time.

"There's a lot being thrown at them, and one of the things is we're not going to wait for them. They've got to catch up, but that's part of their learning process is they have to figure out how to catch up," Rivera said.

"I just wanted to make sure they understood that, and I reminded them we play in 86 days."