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.
"Obviously, I didn't expect one drive and Josh to get knocked out like that," Manziel said. "But that's the way the game goes. You never know what could happen. I thought we responded nice early."
According to the Elias Sports Bureau, the last time a Browns quarterback threw a touchdown pass on third-and-19 or longer was Week 14 of the 1990 season, when Bernie Kosar hit Eric Metcalf for a 31-yard score on third-and-19 against the Oilers.
Manziel committed three turnovers in the second half. He threw an interception in the third quarter and fumbled twice in the fourth. The Jets converted two of those turnovers into touchdowns on their ensuing drives.
Manziel, who also was sacked three times in the second half, took responsibility for the interception.
"That's got to be outside," Manziel said. "It's got to be our ball or nobody's.
"Obviously, the second half leaves a little bit of a bitter taste in your mouth."
Manziel said his right elbow "the majority of the time felt pretty good." He said any discomfort he felt was normal and akin to what he experienced in college at Texas A&M.
"A few times, there were just a couple bites here and there," Manziel said. "Just a little tightness. Other than that, for the majority of the game it really didn't bother me much."
Teammates said Manziel handled himself well and handled the huddle and alignments and play calls correctly. Manziel did not practice from Aug. 23 through Sept. 7 and missed two preseason games, and said he got only a few scout-team reps in practice last week.
"He was coming in here pretty much blind to the whole game plan," tackle Joe Thomas said.
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.
"He's a football player,'' Pettine said of McCown's run. "He's a competitor. In that situation, guys that are mobile, guys that get out in open space, I don't know how many of them would have handled that any differently when they get that close to the end zone.'
"It's a quarterback who's running towards the goal line and has a chance to score a touchdown. I don't know how you coach a competitor, a real football player, to slide there.''
Pettine said McCown is subject to the NFL concussion protocol and can return when cleared.