KANSAS CITY, Mo. -- While the Kansas City Royals headed for St. Louis after an 8-4 loss to the Cardinals on Tuesday night, center fielder Lorenzo Cain was going for a MRI.

Cain suffered a left hamstring injury while running to first base in the seventh inning. Cain was first ruled safe, but upon a review the call was overturned.

By the time the reversal was announced after an 83-second delay, Cain was already being attended to by a trainer and headed for the dugout.

Whether Cain will be going on the disabled list is still unknown.

"I don't know," Royals manager Ned Yost said. "You guys want to know what I don't know. Nobody knows. The doc will look at him and we'll evaluate him tomorrow. I wish I had an answer for you, but it doesn't happen that quick. We'll have a MRI soon as they can get him in. We'll determine how much fluid is on it and what the extent of it is."

The defending World Series champion Royals can ill afford to have another serious injury to a key player. Alex Gordon just returned Saturday after missing 30 games with a fractured wrist, while third baseman Mike Moustakas is out for the season after knee surgery.

"It's tough especially the way the year has been going," Royals first baseman Eric Hosmer said to see Cain go down. "It's happened to quite a few guys. We've just got to figure out a way to get through it. We've got to hope it's not anything serious. It's just an added adversity to a 162-game season. It's time for guys like myself and Gordo to step up and carry the team."

"At the last second it (Cain's hamstring) kind of pops. We've just got to hope for nothing to be torn in there and hopefully it's just a couple of days and we'll get him back sometime on this road trip."

The Royals open an eight-game three-city trip Wednesday, continuing this four-game series with the Cardinals with the final two games at Busch Stadium. The first five games are in National League cities, including three in Philadelphia.

Yost said he is toying with the idea of playing designated hitter Kendrys Morales in the outfield in Philadelphia to keep his bat in the lineup.

The Cardinals will likely be without rookie shortstop Aledmys Diaz for the remainder of the Royals' series. Diaz's right eye was nearly swollen shut Tuesday, looking more like a pugilist than a baseball player. He did not play Tuesday and probably will not return before the weekend.

"I just feel discomfort in my eye with the inflammation," Diaz said. "I don't see very well right now, but thank God it is nothing worse."

Greg Garcia, who started Monday, will likely fill in at shortstop until Diaz returns.

"Soon as we get the swelling down, he should be ready to go," Cardinals manager Mike Matheny said. "They had no concerns structurally or anything else. I don't think it's going to be very long for Diaz."

The Cardinals and Royals have had a rivalry for the Missouri state championship since interleague play began. Well, actually longer.

"This goes back I think a ways to '85 that kind of solidified that," said Matheny, referring to the 1985 World Series that the Royals won in seven games. "When we play these guys, it's always a good matchup. Obviously, they're the team wearing the (World Series) ring from last year. They've brought a young team in, built it from the ground up and did a lot of things right.

"Now it's a young team with some experience and some success. We talk about the different components. They have everything they need to be in a good place to continue to win for a while. I'm sure it's going to be a good matchup for us."

Right-hander Carlos Martinez starts Wednesday for the Cardinals, his first career start against the Royals. He tops the Cardinals with seven victories, 3.02 ERA and .209 opponents' batting average.

Royals right-hander Edinson Volquez is coming off his worst career start, surrendering 12 runs (11 earned) in one inning, plus three batters, Friday to the Astros. George Springer tripled and hit a grand slam in a nine-run first off Volquez.

Yost is not putting any emphasis on the Cardinals series.

"I don't think there is any difference than for any other team, any other game," Yost said. "Each series is important. Playing the Cardinals, it's unique in that we're playing two here and two there. We don't do that anywhere else. I don't look at as a rival or anything like that. I don't think my mindset changes on who we're playing. It's just every series is important."