The rookie sensation from Japan is sidelined with inflammation in his right elbow. He had an MRI in New York and then was headed to Seattle to be examined by Dr. Christopher Ahmad, the team's head physician. Ahmad is in Seattle for a conference.
Yankees manager Joe Girardi acknowledged he's concerned about Tanaka's prognosis.
''You're always worried when you have a guy go back for an elbow MRI,'' he said. ''It would be a big loss. Anytime you lose a starting pitcher, they're hard to replace. And what he's done for us, the innings and the wins and the quality of starts, it's hard to replace.''
The Yankees, who have also lost pitchers CC Sabathia, Michael Pineda and Ivan Nova to injuries, are 46-44 and trail first-place Baltimore by three games in the AL East.
Tanaka allowed five runs and 10 hits - both career highs - over 6 2-3 innings in a 5-3 loss to the Indians on Tuesday night. The All-Star is 12-4 with a 2.51 ERA in 18 starts, but has lost three of his past four outings.
''It goes without saying how important he's been for us,'' Yankees shortstop Derek Jeter said. ''He's been our go-to guy.''
The Yankees signed the 25-year-old Tanaka to a $155 million, seven-year contract in January after he had a standout career in Japan. New York also paid $20 million to his Japanese team, the Rakuten Golden Eagles, for his rights.
Tanaka began the season 11-1 with a 1.99 ERA, but hasn't been as effective in his past four starts, going 1-3 with a 4.25 ERA.
He's allowed eight home runs in his last five games, including two on Tuesday, a two-run shot in the sixth by Nick Swisher that put Cleveland ahead and a solo drive by Michael Brantley in the seventh.
Girardi didn't notice any sign that something was bothering Tanaka during the game.
''There wasn't anything that led us to believe there was anything wrong,'' Girardi said. ''He told the trainers that he just felt some soreness, so we felt it was best to send him back and have an MRI. It's the first time he's complained (of it).''
Tanaka's past two starts came on four days' rest, the regular amount for major league pitchers. He pitched once a week in Japan.
Tanaka has anchored a shaky Yankees rotation. Sabathia might need season-ending knee surgery, and Nova had Tommy John surgery and won't pitch again until next season.
The Yankees are 13-5 in games Tanaka has started. He has recorded 135 strikeouts and walked 19 in 129 1-3 innings. The right-hander has allowed 111 hits and opponents are batting .231 against him. He has given up 15 home runs.
''You just hope and pray that we get good news and it's something minor,'' outfielder Brett Gardner said. ''I worry about anybody, but he's pretty special.''
Tanaka was scheduled to start Sunday against Baltimore, the final game before the All-Star break. Girardi said no decision has been made on who will take Tanaka's spot.