DENVER – Because Monday’s rainout at Coors Field pushed Jake Arrieta’s scheduled start into Tuesday, the Cubs can’t kick their four-man rotation can into next week.

Barring another rainout Wednesday, they’ll need a fifth starter during the weekend series in St. Louis – which could mean calling up right-hander Eddie Butler from Class AAA Iowa to start Friday.

After Brett Anderson (back) went on the disabled list, the Cubs hoped to use off days to stick with four starters until May 20.

Manager Joe Maddon said before Tuesday’s doubleheader he wasn’t ready to say who the Cubs were considering for that start. But it was clear the Cubs were waiting out a questionable forecast for Wednesday in Denver to determine whether the fifth man would be needed then.

Maddon did say he was considering moving Jon Lester back a day, to Saturday, after his 120-pitch outing Sunday.

Butler (1-0 with a 1.17 ERA in five starts for Iowa) last pitched Saturday. It would be his first start since being acquired from the Rockies in a trade in February.

The Cubs’ other option would be lefty Mike Montgomery, who has been stretched in recent relief outings and said he’s prepared to start if asked.