Collins was limited to six games last season after suffering a torn MCL in his right knee in November. He was unable to participate in OTAs last week, the Akron Beacon-Journal reported.
Browns head coach Hue Jackson said Collins is on course to be ready for the start of training camp in July.
"He is definitely making the progress that we feel he needs to make," Jackson told the newspaper.
The 28-year-old Collins recorded 31 tackles, one sack and one interception in 2017 after signing a four-year, $50 million contract.
Collins was a first-time Pro Bowl selection in 2015 with the New England Patriots after amassing 89 tackles, a career-high 5.5 sacks and six passes defensed. He was a member of New England's Super Bowl XLIX-winning team in the 2014-15 campaign.
The Patriots traded Collins to the Browns midway through the 2016 season. In five seasons, Collins has totaled 391 tackles, including 13.5 sacks, in 64 career games.