It was one of the deals the Twins pondered at this week's winter baseball meetings in Dallas.
Romero, 29, originally taken by the Twins in the 21st round of the 1997 free agent draft, is 25-20 lifetime with an ERA of 4.35. He went 4-3 with a 3.47 ERA in 68 appearances for the Twins last season, limiting opposing hitters to a .235 batting average.
Left-handed swingers went only .198 in his 28 1/3 innings.
Romero -- who pitched against the Angels in the 2002 American League Championship Series -- is the southpaw the Angels wanted for their bullpen, to work as a set-up man with right-hander Scot Shields.
Casilla, 21, batted .311 with three home runs, 22 RBI and 49 stolen bases in 98 minor league games in 2005.