Romero was rocked this spring posting a 6.23 ERA and 2.08 WHIP with 10 walks in over 13 innings.
Meanwhile, Happ had a 1.89 ERA in six appearances including five starts. Romero, who is due to make $7.5 million this season, will try to work out his issues in the minors.
Romero, 27, had a rough 2012 season going 9-14 with a 5.77 ERA and 124 strikeouts in 181 innings. He also walked 102 batters and opposing batters hit .282 against him.
“After today’s game we sat down and talked for quite a well,” Blue Jays general manager Alex Anthopoulos said. “Ricky was better today and he’s making strides but he’s not there yet. Ultimately, the more we thought about it, could we have started with him — sure. If he’s not ready and as sharp as he needs to be, we need more time."