Chris Narveson became the 13th different starter when he started on Wednesday against the Pirates, and he was tabbed to fill in after rookie Kendry Flores went on the disabled list with right shoulder tendinitis.
"You know in the offseason you're going to need between eight and 10 starters," Jennings said. "Eight, if you're lucky, and you have good health. Now, we're above 10, to 13.
"It's been tough. It's been tough to deal with the injuries. Eight guys have had DL time. It makes it difficult. You're having to bring some guys here before they're truly ready."
The 13 starters matches the franchise record done previously in 2014 and 2004.
The injuries also have created a situation where Miami is going with four lefties -- Adam Conley, Justin Nicolino, Brad Hand and Narveson.
Another rookie, Jose Urena (left knee contusion) is another candidate to rejoin the rotation in the final month.
The lack of consistency in the rotation has had a trickle-down effect. When starters don't go deep into games, it taxes the bullpen.
Miami is one of four teams without a complete game this season. The Pirates, Mets and Orioles are the others.
Hand came close to going the distance on Tuesday in a 5-2 win over the Pirates. The lefty worked eight-plus innings, but was replaced by A.J. Ramos with two on in the ninth.
"To get that starting pitching to go deep into games is important," Jennings said. "Six innings, you hope almost every night you can get. If you go beyond that, that's a plus. That helps everybody. It keeps the bullpen strong, but it puts guys more in an effective role."
Joe Frisaro is a reporter for MLB.com. He writes a blog, called The Fish Pond. Follow him on Twitter @JoeFrisaro and listen to his podcast. This story was not subject to the approval of Major League Baseball or its clubs.