Janssen will start against Atlanta on Saturday, making his first appearance in the majors since 2007. Romero will join the Blue Jays in time to make his next scheduled start on Tuesday at Baltimore.
Janssen missed all of last season after shoulder surgery in spring training, and suffered a setback this spring. Janssen has a career record of 8-13 with a 3.89 ERA.
Romero went 2-0 with a 1.71 ERA in three starts in April before straining his right oblique.
Snider, batting .242 with three homers and 12 RBIs, was sent down after Boston completed a three-game sweep of the Blue Jays on Thursday. Cecil (2-1, 4.38) allowed five home runs in Wednesday's defeat. Ray (1-2, 4.44) took the loss on Thursday.