The Mets arrived at Friday’s trade deadline with an offense that averaged just 3.85 runs per game. Only two teams were worse: the Texas Rangers and the Pittsburgh Pirates, and both of them are in last place in their divisions.

The Mets, of course, have been leading the National League East for months (every day since May 8, to be precise), with helped from a weak field around them. They held a lead of three and a half games over the Philadelphia Phillies on Friday afternoon, when they added a big bat in an effort to pull away down the stretch.

Javier Baez, a two-time All-Star infielder with 22 homers this season, joined the Mets with starter Trevor Williams and cash in a deal with the Chicago Cubs. The Mets sent the outfield prospect Pete Crow-Armstrong to the Cubs.

Baez, 28, won a Silver Slugger award at second base in 2018 and a Gold Glove at shortstop last year. The Cubs took him ninth overall in the 2011 draft, one spot after Cleveland drafted another middle infielder born in Puerto Rico — Francisco Lindor, now the shortstop for the Mets.

Lindor has been out for two weeks with a strained oblique and is likely to miss at least two more weeks; he has not begun swinging a bat. Baez will take over at shortstop while Lindor recovers, and he offers the versatility to shift to second base (or possibly even third) when Lindor returns.

“I’ve always been a huge fan of what he does as an offensive player, a defensive player, a base runner, all the different things he does on the field,” Mets Manager Luis Rojas said of Baez. “He’s a very exciting player. He can beat a team in all those areas. He’s a guy that can go 0 for 4 but beat you with his glove, or if he gets on base, he can beat you on the basepaths. He’s a guy blessed with a lot of tools to play this game and put the team in a winning situation.”

Baez, who is eligible for free agency after this season, had his best season in 2018, when he batted .290 with 34 homers and an N.L.-best 111 runs batted in. But he has never been a selective hitter, and this season has been an extreme example: Baez has an N.L.-high 131 strikeouts and only 15 walks, with an on-base percentage of .292.

But Baez, a right-handed batter who is hitting .248, brings star power and style to the Mets. He is called “El Mago” — the Magician — for his slick glove work, especially his knack for tag plays. Like several Mets headliners, including Pete Alonso and Marcus Stroman, he is also unafraid to express himself.

In his second-to-last game with the Cubs, on Monday night at Wrigley Field, Baez shouted and gestured as he strutted to first after a game-ending hit off Cincinnati’s Amir Garrett. Baez, who had bickered with Garrett on the field in May, was fined for taunting.

Williams, 29, also had his best year in 2018: a 14-10 record and a 3.11 earned run average in 31 starts for the Pittsburgh Pirates. He has fallen off sharply since then, and though he has a 5.06 E.R.A. in 13 games (12 starts) with the Cubs this season, he shut out the Arizona Diamondbacks for six and a third innings on Sunday, in his final start for Chicago.

Williams — and Rich Hill, acquired from Tampa Bay last week — will bolster a rotation that is missing the ace Jacob deGrom, who is on the injured list with tightness in his right forearm. DeGrom threw 36 pitches in a side session on Thursday, and the Mets have not given a timetable for his return.

Crow-Armstrong, 19, was the Mets’ first-round pick (19th overall) in the 2020 draft, coming out of Harvard-Westlake High School in Los Angeles. He has played six games for Class A St. Lucie, with 10 hits in 24 at-bats.

“It’s not easy for us to give up prospects,” said Sandy Alderson, the Mets’ president, at a Citi Field news conference. “But in this particular case, when you’re a few games ahead in the division on roughly August 1, we needed to do something not only to improve the team but to demonstrate to the players that we had their backs.”