It's a small sample on the relief appearances (mostly in 2011, I think, when the Astros put him in the pen to reduce his innings) but Lyles has been worse as a reliever than starter.

Keep in mind that Lyles has been pitching in front of the worst or nearly the worst defense in the majors (according to advanced fielding metrics) over the past three years. It was pretty obvious last year that he was victimized by fielding misplays more than other Astros' pitchers. (I remember some joking around on threads, wondering if Lyles' position player teammates disliked him.) I think Lyles' mentality comes into play, because it seems like he doesn't handle fielding misplays well (whether it's an official error or just a poor fielding play that doesn't get the out). That's not unusual for young pitchers. (I remember that was a problem for Daryl Kile early in his career.) Lyles presses or loses his confidence when those situations occur (it seems to me). To a great extent, Lyles is a pitch-to-contact guy, and they can be subject to bouts of BABIP misfortune due either to the quality of the defense or just stray "hit 'em where they ain't" batted balls. It seems like these unlucky bloopers tend to occur at the worst time for Lyles. Maybe that reflects the fact that he isn't a big strike out pitcher who can reach back and throw his pitches by batters in tough situations.