HOUSTON — The Houston Astros placed shortstop Jeremy Pena and starting pitcher Tatsuya Imai on the injured list on Monday. Pena was diagnosed with a Grade 1 right hamstring strain, while Imai was sidelined due to arm fatigue.

Pena said his knee tightened up during Friday's game against the Seattle Mariners, but he finished the game. He tried to play on Saturday but exited in the fourth inning and underwent imaging on Sunday. Pena is batting .256 with five runs scored this season. He made his first All-Star team last year after batting .304 with 17 home runs, 62 RBIs, 20 stolen bases and an .840 OPS in 125 games.

Imai exited his Friday start against the Mariners after recording one out. He allowed three runs in one-third of an inning, giving up one hit, four walks and hitting one batter. Imai said he felt fatigued after throwing 37 pitches in that outing. He also said the mound at T-Mobile Park was really hard. He returned to Houston on Saturday.

Imai entered the offseason as the top Japanese starter on the market after posting a 2.14 ERA across two seasons with the Seibu Lions. He agreed to a deal with the Astros one day before his 45-day signing window closed, signing a contract worth up to $63 million.

The injuries to Pena and Imai added to Houston's pitching staff health concerns. Pitchers Hunter Brown and Cristian Javier were placed on the injured list last week after both were diagnosed with Grade 2 shoulder strains. Starter Cody Bolton left Sunday's game in the second inning due to mid-back tightness.

The Astros carried a 1-8 record on their current 10-game road trip heading into Monday's series finale against Seattle. Houston held a 6-8 record after a 9-6 loss to the Mariners to open their three-game series.