I fail to see how leaving Lester in a game rather than going to the worst bullpen in baseball can be a mistake. If Thomas can't get a righthanded hitter out as well as Matt Albers, why is he in camp, let alone on the team.
Lester had already thrown 102 pitches in seven innings, gave up three loud outs in the 7th, and was beginning to have trouble hitting his spots. It's early in the spring, the pitchers are not fully stretched out, the relievers are well rested, and Albers in particular has only thrown 15 pitches, all in one game.
Thomas is a left-handed pitcher with a traditional platoon split. He gives right-handed batters an easy look at the ball, he throws the ball up in the zone, and he gives up flyballs and line drives. He's a LOOGY who failed to get the LHB, and then got left in past his shelf life.
Matt Albers is a right-handed pitcher with a traditional platoon split. He keeps the ball down better than Thomas, has a record of getting groundballs, and is tough on RHB.
It was a no-brainer.