The Mets were riding high on a nine-game winning streak and had pulled to within half a game of the first-place Phillies going into last night’s return from the All-Star break. The Yankees sit in third place, nursing injuries and trailing the defending champion Red Sox and the talk-of-baseball Rays.

Which stories will define New York’s teams in the second half? Let’s have a look.

Yankees First Base/DH

The Yankees reached a deal with Richie Sexson yesterday, adding a right-handed bat to their first base/designated hitter mix. Sexson’s hitting just .218 with diminished power this season, after nearly undercutting the Mendoza line at .205 last year. But he’s also crushed left-handed pitching this year, putting up a 1.045 OPS in 61 at bats against southpaws. The Mariners are eating most of Sexson’s remaining contract, meaning the Yankees only have to pay $390,000 for the rest of the season for Sexson’s services, without giving up the prospects they’d have needed to deliver in a trade for a bat.

The Sexson move comes with the Yankees facing major issues at those two positions. Hideki Matsui is likely out for the season and Jason Giambi has sorely needed a right-handed caddy to take offensive and defensive pressure off him, a job that Wilson Betemit appeared unlikely to handle. For a while the Yankees were playing Jorge Posada at DH, inserting the punchless Jose Molina at catcher to make automatic outs every time through the order. With Johnny Damon hoping to return by the end of next week, the Sexson pickup could at least bring the team close to filling a complete lineup.

Of course, signing Sexson at a minimum price begs the question: If Sexson (so big an offensive sieve that even the lowly Mariners dumped him) was deemed capable enough and cheap enough to DH, then why not Barry Bonds?

Mets Corner Outfield

A healthy Ryan Church would solve a lot of the Mets’ problems. But that’s a big unknown right now, with a projected return to the lineup from concussion problems already pushed back and a firm date still to be determined. Meanwhile, Moises Alou is out for the season. That’s left Fernando Tatis and Endy Chavez to man the Mets’ corner outfield slots. Though Tatis has shown flashes with the bat (.292 AVG/.323 OBA/.433 SLG), he’s a good bet to fall off, given he hadn’t played in a major league game since 2006, and, before that brief stint, since 2003. Chavez never had the bat to be an everyday big league outfielder.

The Mets are rumored to be interested in Xavier Nady and other outfield options. They should be pursuing someone, whether it’s Nady, Bonds, or a sleeper candidate. Expecting a teenaged Fernando Martinez to come up from the minors and be a savior is far-fetched.

Yankees Offense

Find your most optimistic scenario — Alex Rodriguez goes into full rampage mode, Derek Jeter returns to vintage form, Damon comes back and resumes hitting .300-plus with an OBA near .400 — and the Yankees could still be left out of the playoffs if they don’t get better contributions from their two biggest disappointments: Robinson Cano and Melky Cabrera. At least there’s some precedent with Cano. Six weeks into last season, he was hitting a measly .234/.276/.312. He surged from there, ending the season at .306/.353/.488. Cabrera, on the other hand, has always been about strong defense and untapped offensive potential. He’s hitting just .241/.301/.347, a line so weak that his running catches and base runner gun-downs still aren’t enough to make him a significant Yankees asset. Unlike with the 1B/DH situation, though, the Yankees intend to sink or swim with Cano and Cabrera in the lineup.

Mets Second Base

Damion Easley or Luis Castillo, that is the question. With Castillo on the DL, Easley recently rode a 10-game hitting streak, hitting .459 with pop during that stretch. He’s cooled off a bit since then. But Castillo’s never shown any kind of power, even when he was in his prime. Will the Mets stick with Easley when Castillo returns, or go back to the incumbent, accepting the drop in power in favor of better on-base numbers and a better glove? Considering that both players are flawed, the ideal scenario would be to grab an elite second baseman such as the Orioles’ Brian Roberts. Unfortunately, the Mets probably don’t have the minor league depth to pull off that kind of deal, not after committing multiple assets to acquiring Johan Santana, and certainly not if they also want to acquire a legitimate outfielder not named Bonds.

Back of Yankees Rotation

The clock struck midnight long ago on Darrell Rasner. After getting off to a fast start as a rotation fill-in with Ian Kennedy ineffective and Philip Hughes hurt, Rasner’s performance has predictably taken a turn for the worse, with an 8.16 ERA in his past six starts, each time failing to pitch more than five innings. Sidney Ponson’s luck hasn’t run out yet, but years of spectacular failure suggest that the end is nigh. Do the Yankees go after a big-name pitcher such as A.J. Burnett or Erik Bedard, hoping for an ace-level contribution down the stretch and a possible surge to the playoffs? Or do they make do with what they have, welcome Chien-Ming Wang back at full strength for Opening Day 2009, along with a possible co-ace such as C.C. Sabathia? The next two weeks could go a long way toward answering those questions.

Mets Fifth Starter

Can Pedro Martinez hold up? His recent track record makes that an open question, even more so with news that he received a pain-killing injection in his hip yesterday, pushing back his next start. With John Maine and Oliver Perez struggling with consistency and generally not as good as last year, should the real discussion be about the Mets’ no. 5 starter, or about a possible no. 2 starter? Status quo could rule here, given the Mets’ lack of tradable assets, Perez’s recent improvement, and Orlando Hernandez still a possibility within the next few weeks.

At what point do the Yankees become sellers?

They’re five and a half games out in the wild card chase, trailing three teams and barely ahead of another, and six games out in the AL East race, trailing two loaded teams with the minor league depth and trading assets to get even better. It would take a massive second-half surge for the Yanks to have a shot at the postseason, making selling the most prudent option if they’re going to contend in 2009.

Still, this needn’t be a fire sale. Bobby Abreu and Jason Giambi will both be free agents after this season, once the Yankees decline Giambi’s enormous $22 million option for 2009. Trading both players for younger talent could allow the Yankees to get players who could help a lot sooner than the compensatory draft picks they’d receive for a departing free agent. A deal similar to the one that sent Sean Gallagher, Matt Murton, and two others from Chicago to Oakland for Rich Harden and Chad Gaudin could allow the Yankees to improve their young talent base while still giving themselves players who could help a winning team this year, making a late-season charge still possible, albeit unlikely.

Does Carlos Beltran have a 2006-like burst in him?

He’s been the favorite whipping boy of Mets fans, especially with Willie Randolph gone. But if he can return to the form that saw him go .275/.388/.594 in his second season in Queens, Mets fans would surely forgive. Especially since that kind of performance, all else being equal, would nearly ensure a spot in the playoffs.

Mr. Keri (jonahkeri@gmail.com) is a writer for ESPN.com’s Page 2.