Browns quarterback Johnny Manziel and a few other rookies were late to a team meeting on Monday morning.
"He and a few other rookies misread the schedule and they were, I think, fined for being a couple minutes late on Monday morning," Mortensen said.
Mortensen also said the Browns have been pleased with Manziel's work ethic, including his visit to the team's facility on Sunday when the players weren't required to attend following Saturday night's preseason opener.
Browns coach Mike Pettine is expected to choose his Week 1 starter by the third preseason game, which is on Aug. 23.
Manziel was 7-of-11 passing for 63 yards in his NFL debut, and ran for 27 yards on six carries.
- Chris Mascaro