BALTIMORE -- Orioles rookie Jake Arrieta became the latest promising young arm to labor through an ineffective and entirely too short start, a one-time anomaly that has become a cause for increasing concern.

The 24-year-old Arrieta lasted just four innings Sunday -- following a five-inning outing by Saturday's rookie Brian Matusz -- to hand the O's a 10-4 loss in a dreary, 65-minute rain-delayed contest that capped a disappointing 2-8 homestand and secured a series loss.

Those who waited out the seventh-inning storm -- a fraction of the 17,408 announced crowd at Camden Yards -- watched a listless Orioles squad, which optioned popular backup catcher Craig Tatum in favor of an extra arm earlier in the day, continue to tax its bullpen. The O's 'pen, which entered Sunday with three relievers -- Jason Berken, Mark Hendrickson and Matt Albers -- among the American League's top eight in innings pitched, tossed five more in the loss and have pitched a combined 44 innings in 10 second-half games.

"It's pretty embarrassing to go out there and not be able to give your team a quality start," said Arrieta, who has pitched just 9 1/3 innings in his last two starts, allowing 10 earned runs over that stretch.

After pitching out of some early damage, Arrieta loaded the bases with one out in the third on a hit by pitch, single and a walk. He retired Michael Cuddyer on an infield line drive and looked like he might continue to successfully man the tightrope act a while longer, if not for a belt-high 1-0 changeup that Jason Kubel blasted for a grand slam. Kubel sent the 87-mph offspeed pitch 414 feet, depositing it safely in the center-field stands to give the Twins a lead they would never relinquish.

"I let [the Twins] back into too many hitter's counts and they were comfortable in the box," Arrieta said. "I didn't set the tone. I made it tough for the relievers coming in. That's just not acceptable."

Things didn't get much better for Arrieta, who allowed nine baserunners and tossed 57 pitches in the first three innings, and he exited after allowing another run in the fourth. Of the 23 Twins he faced, 13 reached base successfully, bringing reliever Mark Hendrickson -- who was warming in the first inning of Saturday's game -- out for the fifth.

"The bullpen continues to be overused because we're not giving ourselves a chance," a visibly irritated interim manager Juan Samuel said of a topic that has been seemingly been on repeat for the last 10 days.

"The [starters are] not pleased with the outcome of the games. They know the situation that our bullpen is in and they know they have to take us deep. It's no secret. Those guys know it better than anybody and they need to get better."

Excluding Jeremy Guthrie -- who has posted two quality starts and pitched to a 1.98 ERA on the homestand -- the Orioles starters have worked just 36 1/3 innings over eight games, allowing 41 earned runs on 54 hits for a 10.10 ERA. The starting staff has pitched a combined 50 innings on the homestand, just six more than the bullpen, and only the 4-10 Guthrie has an ERA under 5.00.

"The starters have to do a better job to set the tone -- and [Sunday] was a poor example of that," Arrieta said. "There's no excuses. You've got to go out there and execute the game plan. I was very prepared. I just did not execute -- at all. It's upsetting."

Arrieta was hardly alone in poor execution as the Orioles' offense continued their season-long struggle to produce any timely hitting. Following a leadoff double from Corey Patterson and a one-out sacrifice fly from Nick Marakis, the Orioles didn't score again until the game was entirely out of reach, plating three runs off Nick Blackburn to close the gap to six.

"You know we got to do our part, we got to hit" third baseman Miguel Tejada said of an offense that managed just four hits with runners in scoring position in the entire four-game series, two of which came in the bottom of the ninth.

"I know the starting pitching has struggled to at least give us more than five innings [and] that's what we are looking for. But I think we just need to start hitting [earlier] and get the lead and get them more comfortable."

The Twins looked quite cozy, particularly at the plate, as Minnesota twice sent eight batters to the dish in the first six innings, including a pair of back-to-back homers off Hendrickson in a three-run sixth.

"Just hitting around, and having fun," Twins right fielder Jason Kubel said of a series in which Minnesota -- which won three of four -- also got its much-maligned starting rotation on track.

Twins starter Kevin Slowey, who entered Sunday with a 7.80 ERA in three July starts, tossed six innings of one-run baseball for his first road victory since May 17. Slowey posed the 66th quality start against Baltimore in 98 games, and handcuffed an O's offense that looked helpless for most of the homestand.

In their eight second-half losses, the Orioles are just 8-for-69 (.116) with runners in scoring position, and given their struggles as a staff and six errors on the homestand, Baltimore leaves for Toronto misfiring on nearly all cylinders. It's a far cry from the team that posted a four-game sweep in Texas, including an upset over newly acquired Rangers ace Cliff Lee.

"We wanted to play that series here the way we played in Texas, it just didn't happen," Tejada said.

"But it's already over and we are going to start over in Toronto and I hope we have a good road trip."