He has been a member of Detroit's starting staff since 2004, but he has never had this much trouble with his trademark pitch. Following Wednesday's outing in Texas, in which he gave up five home runs in 3 2/3 innings, he is going to work on his arsenal with pitching coach Chuck Hernandez, while manager Jim Leyland uses him in situations out of the bullpen.
"Nate is not going out there with all his weapons, especially his slider," Leyland said Friday afternoon. "So, as we speak, he is in the bullpen."
It was a tough demotion for one of the few remaining connections for the Tigers to their 2003 season. However, he admitted, it did not come as a surprise.
"I knew I was going to have the conversation," Robertson said. "And I knew that it was going to probably lead to one thing or another."
To a large degree, Robertson seemed to understand the decision. As much as the slider has been a trademark during his tenure in Detroit, so was his quiet confidence. Without his normal stuff, his confidence has suffered.
"Skip's got a job to do," Robertson said of Leyland. "And he's got to make the decision for this club. And right now, you can't feel good about me going out there, after what happened in Texas. I mean, I don't feel good about me going out there after what happened in Texas. I know I have better stuff than that. If I don't have anything to throw off of that fastball, then I'm in trouble."
Trouble has been a common experience for Robertson for much of this season. Beyond a 7-10 record, his 6.09 ERA ranks second-highest in the Majors among pitchers with enough innings to qualify for an ERA title. Opponents are batting .318 off of him this season with a .533 slugging percentage, also among the highest among big league starters. He has allowed 195 hits over 149 1/3 innings.
His batting average allowed against left-handers has skyrocketed from .181 two years ago to .315 this season, and his eight home runs allowed to lefties this year matches his total over his previous four seasons combined.
Much of that, he believes, comes off of not having the slider, a pitch that allowed him to pitch aggressively.
"Throughout my career, and especially when it comes to situational pitching, I've always felt like when I've had my legit slider, which is my out pitch, I've felt like I could get out of anything," Robertson said. "And this year, for whatever reason, it's just been inconsistent, off and on. You can just see that in times when I've gone up against a good left-handed bat. It's been a very, very effective pitch. It's been a plus pitch, and it's not a plus pitch right now -- it's inconsistent. When it flattens out, it's very hittable and it's cost me in bad situations in a ballgame.
"I think the biggest thing is getting my stuff back to where it needs to be, because it's not the same. The hit totals and what lefties are doing against me this year pretty much reveal that."
The work with Hernandez has been ongoing this season, and Robertson was optimistic at various points that he had the problems solved. His last victory saw him hold the A's to a run on five hits over 7 2/3 innings, on Aug. 10. However, Oakland had been struggling offensively since the All-Star break, and half its lineup consisted of left-handed batters.
Robertson's woes resumed in his next start against the Orioles, who put up five runs, four earned, over 5 1/3 innings. As the hits piled up, so did the walks -- five of them against Baltimore, followed by four against Texas.
Now, the work with Hernandez will be Robertson's biggest priority without going through the starting cycle every five days. Some of that could carry into the offseason. Robertson also alluded to potential changes in his offseason workout regimen, trying to add flexibility to his frame.
After Wednesday's outing, Leyland said he wanted to check with Robertson to see if he's completely healthy. Robertson told him there was no injury problem. He has dealt with tendinitis in his knee, but that isn't believed to be causing his woes.
Leyland announced the move on Friday afternoon without naming a replacement for Robertson in the rotation. Asked who will start in Robertson's spot on Tuesday against the Indians, Leyland said, "I have no idea."
At this point, however, Leyland said it will not be Dontrelle Willis, who has moved up to Triple-A Toledo in his effort to regain his command after early-season struggles prompted the Tigers to send him to Class A Lakeland in June. Willis gave up three earned runs on eight hits over five innings for the Mud Hens on Wednesday in Norfolk, Va., where he walked five batters and struck out four.
"I think the worst thing to do would be to panic and try to go against everything we've tried to do and replace Nate with Dontrelle," Leyland said. "That would be the easy thing to do, but I think it would be the wrong thing to do right now, and so does [Tigers president and general manager] Dave [Dombrowski]."
Jason Beck is a reporter for MLB.com. This story was not subject to the approval of Major League Baseball or its clubs.