BOSTON -- If Masahiro Tanaka keeps doing this, the New York Yankees have a veritable ace, for whatever that means. On Tuesday night at Fenway Park, Tanaka easily outpitched the ace for the Boston Red Sox, which might mean a whole lot more.

While Tanaka gave up two runs in 7 1/3 stellar frames, Jon Lester was charged with eight runs (three earned) on 11 hits in 4 2/3 innings of a 9-3 loss to the Yankees. He also walked four after entering the contest with 29 strikeouts against four walks in his first four starts.

“I feel like the mistakes I made tonight, they made me pay for them. You can’t make too many mistakes to this lineup,” Lester said. “Had a couple of walks mixed in there, which gave them more and more opportunities to get runners on base and work counts and get you to make mistakes, which gets you in trouble.”

Lester, ever the self-critic, took that sort of approach when he failed to make amends for errors, mistakes and even rulings that went against him.

First there was Jacoby Ellsbury’s drive to center to lead off the game. It caromed off a fan who had reached over the wall, the ball bounding away on the warning track. The umpires, who are allowed to place the runner where they believe he would have ended up if there was no interference, gave Ellsbury a triple, which immediately had Lester a bit miffed.

“I still don’t understand why he ended up at third on fan interference. Maybe I’ll get the whole explanation of it sometime,” Lester said. “But no, that’s part of the game. Sometimes the fans run on the field, sometimes you have distractions. I’ve got to bear down there and make a better pitch to Jeter.”

That would be Yankees shortstop Derek Jeter, who promptly knocked in Ellsbury with a base hit up the middle. Seconds later, catcher A.J. Pierzynski allowed a pitch to get away from him before throwing wildly into center field, Jeter advancing all the way to third. Jeter scored moments later on Carlos Beltran’s base hit to right.

With two outs in the fifth, normally steady first baseman Mike Napoli had a liner go off his glove for an error on what should have been the third out. Four runs would eventually score in the inning, which Lester could not finish.

“Just didn’t make the play,” Napoli said. “It was a little weird. [Yankees catcher Brian] McCann was on first. Kind of lost it for a second. I need to make that play, key situation in the game. They score two runs. Would’ve been the final out.”

Again, despite another knock against a team that entered the night ranked 14th in the American League in defensive efficiency (.666), Lester shouldered the blame.

“I know these guys are out there, we’re all out there busting our butts,” Lester said. “Errors are part of the game. Mistakes are part of the game and tonight they hurt us. Other nights we’ve been able to make up for them and other nights we’ve played exceptionally well. I will never fault a guy for making an error. I know that the effort is always there. I’ve got to do a better job there of picking up Nap. He catches that ball 10 out of 10 times. I’ve got to have a better at-bat right there against Jacoby and pick him up. I didn’t do it.”

Ellsbury’s subsequent two-run double was the big blow in the four-run rally.

While Lester was unable to provide a boost for his teammates, his outing is not the biggest concern. It remains the defense, which has looked much worse than its 14 errors in 21 games would indicate. On that front, there isn’t much to do but work harder.

“We’ve given some extra outs and at this level when you do that, you’re asking for trouble,” manager John Farrell said. “It’s something we continue to address, work out internally. There’s not going to be wholesale changes made, it’s that we have to go out and execute with greater efficiency.”

Kind of like the Yankees, who had no problems making all the plays on defense and saw Tanaka execute brilliantly. He struck out seven and did not walk a batter, improving his strikeout-to-walk ratio to a Cliff Lee-like 35-to-2.

“To his credit, he throws a lot of strikes. He never gave in,” Farrell said. “His split is one that presents itself to the strike zone and forces guys to commit. And with late action, you’re going to get some swing and miss and some mishits. The overall walks issued speaks to his strike-throwing ability. He forced us to swing the bat early in the count at times to try to get something going. He pitched a very good game tonight.”

Tanaka has a little ways to go to become firmly established as a top-of-the rotation guy, perhaps proving that he can dominate an opponent the second time he sees them. Lester’s already there. However, in part because the former received support and the latter did not, their initial encounter was a mismatch.