It appeared Bell was poised to make the start for injured left-hander Joe Saunders after he was pulled after just two scoreless innings in his start for Triple-A Salt Lake on Saturday, but no official announcement was made until Monday.
Scioscia said he picked Bell to start over in-house candidates Matt Palmer and Shane Loux, who will remain in the bullpen in middle relief.
"I think there are a number of reasons [to start Bell]," Scioscia said. "Our bullpen, the continuity down there is important. And I think Trevor has shown the command and the stuff to do at least or more than our options we have here. And that will keep a little more depth in our 'pen."
Bell, 22, was 3-4 with a 3.15 ERA in 11 starts with Salt Lake after going 4-3 with a 2.23 ERA to start the season at Double-A Arkansas.
However, Bell saw his strikeout rate decrease when he advanced. He struck out 38 batters in 71 1/3 innings with Salt Lake, after striking out 51 in 68 2/3 frames with Arkansas.
But Scioscia said he's confident the young right-hander has the stuff for the Majors right now.
"I think he's ready," Scioscia said. "I think he's the best option for the staff to move forward from here."
Scioscia also made it clear that Bell will stick around for at least one more turn in the rotation while Saunders is on the 15-day disabled list.
"It'll definitely be more than one shot, but it's tough to look two to three weeks down the road," Scioscia said. "But it'll definitely be more than one start."
Because Bell currently is not on the 40-man roster, the Angels could have a potentially difficult decision to make.
One easy way to make room on a 40-man roster is to shift a player from the 15-day disabled list to the 60-day DL. But only Torii Hunter and Saunders are on the 15-day DL, and it appears unlikely either would be headed to the 60-day DL, especially with Hunter hoping to rejoin the team this weekend in Baltimore.
Scioscia wouldn't comment yet on how the Angels will make room for Bell on the roster.
Rhett Bollinger is a reporter for MLB.com. This story was not subject to the approval of Major League Baseball or its clubs.