When you’re riding in the back end of a pitching rotation — especially one as volatile as the Phillies — things can tend to get a bit bumpy.   

Jerome Williams has been in Sean O’Sullivan’s and now Severino Gonzalez’ shoes and says they’re usually not a comfortable fit. 

When you’re the ace like Cole Hamels, everything’s laid out nicely. It’s almost etched in stone that you’ll take the mound every fifth day. But when you’re the No. 4 or 5 guy there’s no stability. You’re never sure when — or even if — your turn will come and often must adjust on the fly.

" I’ve been through that," said the 33-year-old Williams, a former first round draft pick now pitching for his seventh team. "I know how to keep myself prepared. It’s imbedded in my brain so I can do it without blinking."

But not always.  

"It is tough when you’re a young guy," said Williams, who lost to the Phillies in his April 26, 2003 debut, then watched the Phils’ Kevin Millwood throw a no-hitter the following day. "At that time I didn’t know how to deal with it. It’s tough to swallow because you’re used to being that No. 1 or 2 guy in Triple-A and all of a sudden you’re the five-guy in the majors. You’ve got to re-learn some things. 

"You just try and watch and learn from veterans."

While the fact you’re in "The Show" beats the alternative, it’s often a thankless job. The 22-year-old  Gonzalez gets the call in Colorado Tuesday simply because Chad Billingsley went down with a shoulder sprain. A few weeks back Gonzalez was shelled in his major league debut, but pitched well enough to get the win in Miami five days later.

His reward: a ticket back to Lehigh Valley.

"It’s part of the game," said the 30-year-old Billingsley, hours before learning his comeback from a torn flexor tendon and Tommy John surgery would be aborted for at least a month. "Guys go on the DL or someone gets healthy and they have to make a move. You can’t be naïve. You understand it’s a game and a business. As a ballplayer you can only control what you do. Keep performing for them and make a good impression, so that next time you’re gonna be that guy."

O’Sullivan knows that all too well.  

"You fly into a situation where you haven’t been and maybe don’t have the same rapport with the catcher," said the 27-year-old O’Sullivan, who went more than four calendar years between big league wins. "And sometimes you have one opportunity to try and prove something. Sometimes you’re gonna show up, pitch and go back down regardless. So it’s more of a mental than physical grind. You just try and take it best you can and prepare a day at a time. But it’s really tough.’’

And as constant reminder, no matter how long you’ve established yourself, it can suddenly change.   Considering the Phils went to spring training with a expected rotation of Hamels, Cliff Lee, Aaron Harang, Williams and David Buchanan and have already had to call on Billingsley, O’Sullivan and Gonzalez shows how tenuous putting a rotation together can be.

"It’s been awhile, but I’ve been in that position," said the 37-year-old Harang, referring to his early years. "I came to realize with Oakland when we had (Mark) Mulder, (Tim) Hudson and (Barry) Zito every single night they’d go 7, 8 or 9 innings. I was starting, but I was also long guy in the pen. If the fifth day would come and we’d have an off-day they’d bump me around.’’

That’s the typical life when you’re in the back end of the rotation, which Williams, O’Sullivan have already learned and Gonzalez is quickly finding out. Here today. As for tomorrow… we’ll have to see.

Cole Hamels, you have no idea how lucky you are.