He won 19 games a year ago and was the starter for the National League in the All-Star game. Jimenez has struggled to regain that form, losing again as the Rockies fell to the Pittsburgh Pirates on Sunday.
"Your guess is as good as mine," perplexed Colorado manager Jim Tracy said. "If I had that answer, we'd have fixed it and we would have fixed it a heck of a lot sooner than on May 1."
Charlie Morton (3-1) pitched into the sixth inning for his third win, and Garrett Jones had two hits and three RBIs for the Pirates. Meanwhile, Jimenez (0-2) threw three wild pitches, tying a club record, and his ERA rose to 7.20 through four starts. He allowed four runs on six hits, struck out six and walked four.
"I don't know what is going wrong," Jimenez said. "It's probably a problem with my mechanics. I had been trying to get my velocity back and right now I have to improve my velocity and my location at the same time."
Tracy agreed that Jimenez's velocity is unpredictable.
"At times, we're seeing 96 or 97 mph go in there," he said, "and in between that, you see any where from 89 to 93."
He needed 88 pitches to get through four innings in his shortest outing of the season. Last year, Jimenez failed to pitch into the sixth just twice in 33 starts and dominated hitters en route to a 15-1 record at the All-Star break.
"My velocity wasn't that bad today," Jimenez said. "I just didn't have my control, especially with my slider. I kept bouncing it and bouncing it into the hitters."
A cracked cuticle on his right thumb landed Jimenez on the 15-day disabled list after opening day and he has struggled with his control in his four starts.
"Are we concerned? Certainly we are concerned," Tracy said. "The concern is we are 17-9 and the ace of your staff has not thrown a pitch in the sixth inning and you don't have a win from him."
Tracy insists that there is nothing physically wrong with Jimenez.
"With all the conversations we've had with him I really don't," Tracy said. "He would not be pitching and we aren't getting any answers from him that he's dealing with any sort of pain."
Dexter Fowler and Todd Helton had two hits each for the Rockies.
"We had some misfortune today in not capitalizing with runners in scoring position," Fowler said. "I'm certain we will be able to turn that around just like I'm sure Ubaldo will get it turned around."
Xavier Paul, Andrew McCutchen, Lyle Overbay and Pedro Alvarez also had two hits each for the Pirates, who took two of three at Coors Field. Pittsburgh won its fourth road series of the season, matching its total from the 2010 season.
The Pirates jumped on Jimenez in the second inning. Ryan Doumit led off with a walk, Alvarez singled to put runners at the corners and Ronny Cedeno's bunt down the first base line gave Pittsburgh a 1-0 lead.
Jimenez retired the next two batters before Paul tripled to right-center and Jones doubled to center to make it 4-0.
"My mindset going into the game was just have fun," Paul said. "To come up with a big hit, it feels great."
The Rockies broke through in the third on Helton's two-out single that scored Fowler from second to make it 4-1. Ryan Spilborghs cut the lead to 4-2 with a pinch-hit single to score Seth Smith in the fourth.
The Pirates broke open the game with a four-run fifth against reliever Esmil Rogers. Morton knocked in his second career run on a sacrifice fly and Jones had a two-run single to make it 8-2.
"The key to today was we pitched poorly the first five innings of the game," Tracy said. "When you throw 128 pitches between two guys in the first five innings of the game, the ebb and flow of the game, the way you want it to be and the way we want to go is lost."
Morton was cruising until the sixth when Smith led off with a single and Chris Iannetta drew a one-out walk. Michael Crotta relieved and walked pinch-hitter Jonathan Herrera to load the bases. Fowler followed with a double to make it 8-4, but Joe Beimel came on to strike out Alfredo Amezaga on three pitches and retire Helton on a deep fly to left.
Notes: Four pitchers held the Colorado record with three wild pitches. Jimenez was the last to do it, on April 25, 2008. ... Pirates OF Jose Tabata (right hamstring) was held out of the lineup.