Velasco, 29, started 11 games at center for the Steelers last season before rupturing his achilles tendon in week 13.
The 6-foot-3, 308-pound Velasco spent his first two seasons with Tennessee on the practice squad after going undrafted out of Georgia in 2008. He signed with Pittsburgh after starting 16 games -- three at guard -- for the Titans in 2012.
Matthews, the son of Hall of Fame offensive lineman Bruce Matthews, went undrafted out of Texas A&M in 2010. He appeared in 14 games with Tennessee in 2012 but was waived by the Redskins in training camp last year.
The Panthers went 12-4 and finished first in the NFC South last season before falling to the 49ers in the divisional round of the playoffs.
- Chris Johnson