A converted pitcher now playing the outfield, Loewen hit his first major league homer to help the Toronto Blue Jays beat the Orioles 6-5.
"I know so many of the guys over there, it was special to do it against them," said Loewen, who was both a pitcher and position player in high school and junior college.
Jose Bautista drove in the tiebreaking run with a seventh-inning sacrifice fly, sending Baltimore to its 29th loss in its last 34 games in Toronto dating to June 2008.
Cooper, who has come up through Toronto's minor league ranks alongside Loewen, said his teammate's switch from power arm to power hitter is "unreal."
"I remember when he first made the transition and it's night and day," Cooper said. "It's amazing how quickly he's picked up hitting. There's a lot of things to come. He's got a lot of power."
Hunter, who gave up Loewen's shot, expects to be reminded of it for some time to come.
"We have the same agent so I'm probably going to hear about this for the rest of the time that we're together," Hunter said. "He's gone through a lot of adversity in the game of baseball. To come back and be productive in the big leagues, not only as a pitcher but coming back at a different position is just ridiculous. Give him a lot of credit."
Baltimore's first-round draft pick in 2003, Loewen pitched in 35 games for the Orioles between 2006 and 2008 before giving up pitching because of recurring stress fractures in his elbow. He signed a minor league contract with Toronto that winter and began his comeback as a position player, getting called up to the majors last Wednesday after finishing the year at Triple-A Las Vegas with a .306 average, 17 homers and 86 RBIs.
Cooper led off the inning with a drive to right, his second. J.P. Arencibia, whose ninth-inning single gave Toronto a 5-4 win Saturday, hit a deep drive to center that Matt Angle caught at the wall. Loewen followed with a shot to almost the same spot, but that one cleared the fence for a tying homer.
"I didn't really think he got it that good but he's a big, strong guy and he got it out to center field, so I tip my hat to him," Orioles infielder Chris Davis said.
Vladimir Guerrero's sacrifice fly gave Baltimore a 1-0 lead in the first but Toronto answered with two unearned runs in the bottom half. After McCoy reached on an error and Eric Thames on a single, Bautista hit an RBI single and Brett Lawrie's groundout drove in another run.
The Orioles used a walk and three straight singles to tie it in the second, with Angle's base hit scoring Davis.
Toronto reclaimed the lead in the third. McCoy singled, snapping an 0-for-15 slump, took second on a groundout and moved to third on a fly ball before scoring on Adam Lind's single.
The Orioles answered in the fourth, chasing Toronto starter Dustin McGowan after he opened the inning with back-to-back walks. Carlos Villanueva came on and struck out Roberto Andino, then got Angle on a groundout, with the runners advancing to second and third. J.J. Hardy was walked intentionally to bring up Nick Markakis, who lined a two-run single to right.
Making his first start since July 8, 2008, also against Baltimore, McGowan struggled with his control, allowing three runs and four hits in three-plus innings. He walked five and struck out none.
"This is a rehab year. There's going to be peaks and valleys with arm strength along the way," Blue Jays manager John Farrell said.
Villanueva pitched two innings of relief. Camp, Carreno and Casey Janssen each worked one inning before Francisco closed it out.
Hunter, who has not won in three starts, allowed six runs and nine hits in 6 1/3 innings. He walked none and struck out five.
Orioles CF Adam Jones was scratched for the second straight game with a sore left ankle. Jones fouled a pitch off his ankle Friday. ... Toronto SS Yunel Escobar, who left Saturday's game after being hit on the left elbow by a pitch, did not start. He is day-to-day. ... Cooper's homer was Toronto's major league-leading 43rd by a rookie. ... Orioles RHP Jason Berken (strained right forearm) is expected to come off the disabled list when the team returns home Monday to begin a three-game series with Tampa Bay. ... To commemorate the 10th anniversary of Sept. 11, members of the Toronto Police and Toronto Fire Department presented the colors and a Toronto firefighter sang the national anthems while the players lined the basepaths. In lieu of a first pitch, Blue Jays LHP Ricky Romero placed the ball on the mound before a moment of silence. Players wore a uniform patch and two special crests were painted on the field.