Riley was one of the most mismanaged prospects ever. He had the unfortunate timing of coming up in an organization that was in disarray from the top on down. When you add in his lack of maturity at the time, it was a bomb just waiting to happen.
From my article back in 2002
One of the worst cases has to be the mishandling of Matt Riley. Riley was a 19-year old left-hander who had made his way to Double-A thanks to a mid-90s fastball and a knee-buckling breaking ball. However, Riley was also a kid who had some very real maturity issues that required special handling. During one start in late June, Riley threw a pitch and then fell off the mound holding his elbow. The Orioles called it a strained elbow and put him on the seven-day disabled list. Riley missed two starts and was activated, but he was never the same pitcher again. After being activated, Riley posted a 5.20 ERA (26ER/45IP) in his last eight starts, including an 0-3 record with a 5.96 ERA (15ER/22.2IP) in his last four starts. His velocity had dropped off noticeably and by mid-August, he was topping out at only 89MPH. Despite the obvious signs of something wrong, it didn't stop the Orioles from rushing the first true left-handed pitching prospect that organization had seen since Arthur Rhodes to the major leagues. After meeting with Riley and his agent, Frank Wren decided to promote Riley to the major league for a September look. The results were predictable: Riley made three starts, posting a 7.36 ERA with 13 walks and just six strikeouts in 11 innings, before the Orioles mercifully shut him down.
The Orioles didn't really monitor Riley that offseason, and he showed up in the major league camp out of shape, but with an attitude that could be summed up by his vanity license plate, "24kt arm." His frequent lateness and attitude rubbed the Orioles' established players the wrong way, and Riley was soon ostracized from the rest of the team. That didn't stop the Orioles from keeping him in the major league camp, where he saw one inning's work over three weeks of the exhibition season. Riley was eventually sent to the minor league camp across the state in Sarasota, from where he was later assigned to Rochester despite still being out of shape. With no legs under him, Riley was rocked at Rochester and eventually put on the DL with arm soreness. When he returned from the DL he was sent to Bowie where he was put into the bullpen until he regained his strength. He pitched poorly for most of the 2000 season before feeling a sharp pain in his elbow after throwing a pitch in a game at the end of August. Riley had torn a tendon in his left elbow and underwent Tommy John Surgery. He would miss the entire 2001 season.
Despite missing that entire season, Riley wasn't brought back slowly the way the Cardinals handled Matt Morris, another young pitcher who missed a year with Tommy John surgery. No, Riley was placed into the Baysox rotation where he compiled 109 innings in 22 starts, posting a 4-10 record and a 6.34 ERA.