But he doesn't seem overly worried.
The Panthers have been searching for a replacement for longtime left tackle Jordan Gross, who retired earlier this offseason.
Byron Bell, last year's starting right tackle, and Nate Chandler, a converted defensive tackle, have been working as the starting tackles during OTAs. They've been flip-flopping from left to right tackle, depending on the day.
Neither has played left tackle before in the NFL.
Rivera said veteran Garry Williams, who is coming off a knee injury, could also figure into the mix once healthy.
''We've not decided on a lot of positions yet,'' Rivera said. ''The biggest thing we have right now is competition and guys working. ... We'll see how we are once we get into training camp.''
Newton hasn't participated in OTAs except to throw on the side.
He said he could be ready for minicamp next week, but at the very least should be completely healthy when training camp begins July 24.
''Come game time, it's no doubt in my mind that every single person will be ready to go. That is what OTAs and minicamp are designed for - to get these guys prepared, to get myself prepared,'' Newton said. ''When that ball is snapped I have full confidence in Nate Chandler and Byron Bell that they will get the job done.''
While many teams invest millions in protecting their quarterback's blindside, the Panthers seem to be taking a chance with lower-priced players who haven't played the position.
Bell and Chandler will make a combined $2.6 million this season.
Both players insist they don't have a preference on which position they play.
''We're both getting an equal opportunity to win a job, which is nice,'' Chandler said.
Said Bell: ''I really don't care. Either position, wherever coach sees being the best fit for either one of us is fine. I'm just trying to win football games. If that means left tackle, right tackle or on the bench, wherever the team needs me is fine.''
Filling Gross' shoes won't be easy.
A three-time Pro Bowl selection, Gross started 167 games during his 10 seasons with the Panthers and was a member of the Super Bowl team as a rookie in 2003.
The 6-foot-4, 315-pound Chandler is an intriguing story.
He was signed as an undrafted free agent in 2012 out of UCLA. He started his career with the Bruins as an offensive lineman, but later moved to tight end as a sophomore and then to defensive tackle for his final two seasons.
When he arrived in Carolina he saw spot duty at defensive tackle a rookie, playing in 10 games with five tackles in 2012. But coaches thought with Chandler's combination of quickness and size he might be more valuable on the offensive line, so they moved him there last year in training camp.
Chandler started eight games in 2013 - six at right guard and two at right tackle - due to injuries.
Bell was also undrafted.
He came out of New Mexico in 2011 and was thrust in the mix as a rookie. He's started 41 games over the past three seasons for Carolina - all but one of those at right tackle.
''I'm left handed, so I'll feel comfortable over there,'' said Bell, who started three years at left tackle at New Mexico. ''But if it is right tackle, I'm certainly fine over there.
''The big thing is we as an offensive line have to do is go in with the mindset that we have to keep No. 1 (Newton) upright. And if you have a bad play, you have to forget about and move on the next one because that (pass rusher) will be coming again.''