The two-way star has been battling inflammation and soreness in the knee for weeks and skipped the Major League Baseball All-Star Game to receive treatment and rest.
He had remained in the Dodgers' batting lineup, however, with the team saying the knee was only a problem when he pitches.
Ohtani hasn't pitched in a game since July 3, and this week he postponed a planned bullpen throwing session over what Roberts called "regression" of the knee injury.
Roberts insisted before Thursday's game against the Seattle Mariners that he didn't think the problem would keep Ohtani from his hitting duties long-term.
"I expected him to play," Roberts said, adding that he was "very confident" that Ohtani would be back in the lineup on Friday.
Roberts, who added that Ohtani had been on base seven times over the previous two nights could have possibly contributed to the soreness, said it seemed too soon to consider putting Ohtani on the injured list and sidelining him for more than a week.
"That's a hard question and I understand the thought behind it. But it's gotten better over the last few weeks, markedly," Roberts said. "He's been a very productive player...You trust him. He feels like he can go.
"He's not 100 percent. Very few players are 100 percent. Then what is it? Is it 70 percent, is it 80 percent, is it 90 percent? For me, there's still that conversation, where he understands the most important part of the season is to come, and he understands his value with our team."