HOUSTON — The last thing the Mets needed was another injury to a starting pitcher. Carlos Carrasco exited his start with lower back tightness in the third inning of the Mets’ 5-3 loss to the Astros on Wednesday afternoon at Minute Maid Park. Carrasco gave up five earned runs on four hits, including three home runs, and struck out two over 2 1/3 innings and 53 pitches in his 14th start of the season Wednesday. Credit to Mets relievers Yoan Lopez, Tommy Hunter, Joely Rodriguez and Edwin Diaz who combined to pitch 5 2/3 scoreless innings after Carrasco’s early exit and kept the team within striki…