The injury was initially announced as right hamstring tightness.
McClendon said Cruz's pelvic bone briefly popped out of place, but that trainers had popped it back shortly thereafter. He said he was told the injury was neither serious nor long term.
"I'm not sure how to describe it, but his pelvic bone kind of popped out of place and he had some discomfort," McClendon said. "He's OK now but we'll see how he feels tomorrow."
The 34-year-old slugger popped out to Royals first baseman Eric Hosmer in the first inning and then grounded sharply to Hosmer to end the fourth but was removed from the game prior to his third at-bat.
When asked about how the injury happened, McClendon said he did not know specifics and that Cruz came to him about the injury.
"Your guess is as good as mine," McClendon said. "I have no idea. He came to me right before his last at-bat and said he couldn't go."
Dustin Ackley pinch-hit for Cruz with one out in the seventh inning. He worked a full count before swinging and missing at a 100-mph offering from Royals reliever Kelvin Herrera for the second out. Third baseman Kyle Seager then lined to left to leave center fielder Austin Jackson stranded at third after a leadoff double.
Ackley stayed in the game in the eighth, taking over for Seth Smith in left field as Smith moved to right field to replace Cruz.
In the three games prior to Monday night's contest, Cruz was 4-for-8 with 3 RBIs. In the Mariners' 6-3 win over the Astros on Saturday night, Cruz hit his 19th home run of the season, his first since May 27.