Carolina left tackle Matt Kalil can attest to that.
The Panthers invested $55 million over five years this offseason in Kalil to protect quarterback Cam Newton's blindside and keep his jersey clean. But through three games Kalil has allowed four sacks, which is tied for the most in the league among left tackles, according to Pro Football Focus.
He allowed two sacks last week in a 34-13 loss to New Orleans. One of those came when he blocked the wrong player and allowed Saints defensive end Cam Jordan to race in untouched and level quarterback Cam Newton in the red zone.
Rivera said overall Kalil has done a "pretty good job," but is still learning to play with his new teammates including Newton, who takes a deeper drop in the pocket than most QBs and tends to hold the ball a little longer.
But Rivera is quick to add, "Pretty soon we're going to run out of time, so that learning curve has to be now."
The Panthers need Kalil at his best on Sunday when they travel to face the defending Super Bowl champion New England Patriots.
"I feel like it's going well and I'm getting better each game," Kalil said. "Getting back into it after having a year off so that is always an adjustment. But things are going well."
Kalil missed nearly all of last season with a hip injury that required surgery.
He's still trying to break old habits that he picked up while overcompensating for that injury. He's been working closely with offensive line coach John Matsko on his fundamentals and feels like he's making good strides.
"It's one thing to do them in a practice but you get in the game and get tired and fatigued and you go back to your old pattern," Kalil said. "So I have been getting away from that and getting more comfortable. I've been sharpening things up and getting to a point where I am feeling comfortable again."
Rivera doesn't seem overly concerned about Kalil, who was a Pro Bowl selection for the Minnesota Vikings as a rookie in 2012.
"When you look at what we do offensively it puts a lot of pressure on the left tackle," Rivera said. "He doesn't get a lot of help out there and once he settles in and gets comfortable I think we're going to be fine."
Kalil came into the season with the goal of not allowing a sack all year.
He was frustrated over allowing the sack to Jordan, a play in which he simply misunderstood who he was supposed to block. He said he tried to put it behind him as quickly as possible.
"In the game you have to act like nothing happened. You can't let crap build up," Kalil said. "We are human beings and stuff happens in the game. It wouldn't be the first time an offensive lineman went the wrong way. I just have to learn from it and obviously that will probably never happen again the rest of the season."
Carolina's offense has struggled but it's hardly all Kalil's fault.
Newton is coming off right shoulder surgery and has struggled with his accuracy. He's thrown twice as many interceptions as touchdowns.
The good news for the Panthers is it appears Kelvin Benjamin will play against the Patriots.
Benjamin injured his knee on the final play of the first quarter against the Saints and did not return. He returned to practice on Thursday and participated in all drills. Barring a setback, Rivera expects he'll play.
However, Kalil's older brother, center Ryan Kalil, remains out of practice with a neck injury. The elder Kalil has missed the last two games and has not practiced this week.
For more NFL coverage: http://www.pro32.ap.org and http://www.twitter.com/AP_NFL
Copyright 2018 The Associated Press. All rights reserved. This material may not be published, broadcast, rewritten or redistributed.