The Mariners also announced right-hander Carlos Silva has been diagnosed with an impingement and fraying of the labrum and rotator cuff in his shoulder. He is expected to get a second opinion from Mets medical director Dr. David Altchek this week.
Silva went on the disabled list May 7 with right shoulder inflammation and had an MRI on Monday. He is 1-3 with an 8.48 ERA in six starts this season.
Beltre, who was in his normal cleanup spot in the lineup, took extra batting practice before the game against the Los Angeles Angels, going to the opposite field. Wakamatsu and hitting coach Alan Cockrell watched the entire session.
The 30-year-old Beltre entered Tuesday night's game in an 0-for-13 slump, batting .211 with two homers and 15 RBIs this season. The Mariners lost 10-6 to the Angels on Monday night.
"A lot of his struggles, as I said before, is not lack of focus or care," Wakamatsu said. "It's what we talked about yesterday is let's take a little smarter approach about it and he had more one-on-one stuff.
"Sometimes guys go through a slump and they want to get away. I just challenged him instead of trying to forget it, let's work on some things that we think he needs to work on and he's all for that."
Another regular, shortstop Yuniesky Betancourt, was replaced in the Tuesday's lineup by Ronny Cedeno. Wakamatsu benched Betancourt for two games in Texas last week in an effort to get him to focus on what the team needs from him.
The Mariners want Betancourt to be more patient and show more discipline at the plate. He also has been struggling defensively.
"It's not that I am sending any messages, but it's more about the level of play expectations," Wakamatsu said. "That's one of the main reasons Ronny is playing short today.
"If we are going to go forward and do what we want to do here going forward, the level of expectation has to go higher and have to make adjustments if mistakes are made."