Manziel took over after starter Josh McCown suffered a concussion late in the first quarter of Sunday's season-opening loss against the New York Jets. Manziel threw a second-quarter touchdown pass that gave the Browns a 7-0 lead, but had three second-half turnovers in Cleveland's 31-10 loss.
"I think that's a tough spot given our practice rules and how little work that No. 2s actually get (in practice)," Browns coach Mike Pettine said. "I thought he did some good things."
But Pettine also added that Manziel has "got to be more protective of the ball."
Manziel's touchdown came on his second pass. He found Travis Benjamin on a go route on third-and-long for a 54-yard score. It was the first NFL touchdown pass for the Heisman Trophy winner.
McCown left after being hit by Jets defenders Calvin Pryor and Demario Davis just short of the goal line on third-and-goal from the New York 14. Davis' helmet hit the side of McCown's helmet. McCown fumbled on the play, turning a promising opening drive into a turnover as the ball bounced out of the end zone for a touchback. McCown was 5-of-8 for 49 yards before leaving and had three rushes for 23 yards. Cleveland hosts Tennessee in a Week 2 matchup. McCown will have to go through the league's protocol and Manziel would start if McCown can't be cleared.