Of course, what the four players previously mentioned have in common is the fact that all were acquired during the season and only Piazza was not added on the exact day of the respective season's trading deadline. In all cases, the Mets needed both leadership and offense and were able to get that with the deals that were made. Although other trades have been made, the Cespedes deal brings to memory the impact of the other three. Looking at the numbers (and some other facts), let us review the following trades and how they impacted the immediate and distant future of the franchise. After that, it may be easier to conclude which player had the biggest impact.
Of course, the Cespedes book has not been completely written. The Mets are still entering the second week of September and Cespedes is an impending free agent after the season with an odd clause in his contract that gave the Mets until 5 days after the completion of the World Series to re-sign him (which has since been restructured.) That takes us back to the Piazza trade, where a similar fear existed that the Mets would not re-sign Mike. While the expectation at this point that the Mets will hold off the Nationals for the NL East title this year, they have yet to clinch. However, there is no doubt that Yoenis Cespedes has been the key to the Mets rebirth as a first division club. Since August 1 (Cespedes' first game in NY), the Mets are 24-11 and are 16 games over .500 for the first time since the last game at Shea Stadium on September 28, 2008.
Cespedes cost the Mets former 1st round draft pick (and Passed Ball Show guest) RHP Michael Fulmer and RHP Luis Cessa. To this point, Cespedes has hit .307 with a .354 OBP, .660 SLG for a 1.014 OPS. In 35 games, he has hit 13 HR and driven in 34. The Mets spent the first three months of the season among the worst in all of baseball in runs scored, extra base hits, batting average and on base percentage. The acquisition of Cespedes has impacted the way the rest of the lineup is pitched to and has made the Mets more of a threat as an offensive team.
The deal for Clendenon is probably the one that is the most comparable to the Cespedes trade. Similar to the Mets of 2015, the 1969 team was carried by their young starting pitching. Because of the position the Mets were in (of course, it was much more of a surprise in 1969), the team needed a proven bat to put in the middle of its batting order. Donn Clendenon had spent the first 8 seasons of his big league career playing for the Pittsburgh Pirates. The Pirates left Clendenon unprotected in the 1968 expansion draft, not expecting him to be taken by any of the four incoming teams (San Diego, Montreal, Seattle, Kansas City). The Expos selected him with the hopes of trading him before the season started. They had their sights on La Grande Orange, Rusty Staub, who at that time was playing for the Houston Astros.
A trade was consummated, but it was Clendenon who did not want to go to Houston. He did not want to play for former Pirates manager Harry Walker, who happened to be behind the bench for the Astros. Clendenon would play for the Expos until the trading deadline of that season. On June 15, the Mets acquired Clendenon from the Expos for Steve Renko and Kevin Collins as well as minor leaguers Jay Carden, David Colon and Terry Daley. Clendenon had some big hits for the Mets down the stretch for the Mets in 1969, but overall, his numbers didn't stand out. Donn hit .252, 12, 37 in 72 games with a .777 OPS. However, it was the 1969 World Series where Clendenon starred, as he hit .357 (5-14) with 3 HR and a 1.071 OPS while winning the WS MVP- his only postseason appearance as a Mets player as well as his career. Clendenon stayed with the Mets through 1971 and in 281 regular season games, hit .267, 45, 171 with a .797 OPS.
The reason the Mets trade for Hernandez is different from the other three is because the Mets traded for Hernandez in 1983 where the team was not ready to compete for a World Series Championship. In fact, the 1983 Mets finished at 68-94, the year before they hired Davey Johnson as manager. A reasonable argument that the acquisition of Hernandez was the turning point in the franchise's success was how his presence was the change in the culture in the clubhouse. Similar to Tom Seaver coming up with the 1967 and 1968, Hernandez' inflicted an attitude of winning to a team that was not used to it. From that perspective, a case could be made that Hernandez had the biggest impact on the franchise for the long term.
On June 15, 1983, the Mets acquired Hernandez from the St Louis Cardinals in exchange for RHPs Neil Allen and Rick Ownbey. For the rest of the 1983 season, Hernandez slashed .306, .424, .434 with 9 HR and 37 RBI. It took the entire 1984 and 1985 seasons for Hernandez' presence in the clubhouse to translate into postseason baseball, similar to Seaver in 1967 and 1968. For his career with the Mets, Hernandez hit .297 with a .816 OPS, 80 HR and 468 RBI in 880 games. Keith will be forever remembered as one of the captains, along with Gary Carter, who were responsible for bringing the World Series Championship back to the New York National League team in 1986.
The trade made for Piazza saw the Mets in a similar type of dire straights to the most recent deal for Cespedes. The Mets had brought in veteran players to mix with some of their youth, but needed a star player to put into the center of their lineup. Piazza changed the tune of the clubhouse, similar to Hernandez and just like Cespedes, made the Mets lineup much more formidable. However, this trade was made on May 22- well before the trading deadline of July 31st. (Both Clendenon and Hernandez were traded on the prior deadline, which had been set at June 15th for years.) The Mets sent OF Preston Wilson, pitcher Ed Yarnall and minor leaguer Geoff Goetz to the Marlins, who had just acquired him in a blockbuster deal with the Los Angeles Dodgers.
Mike played 109 games to finish the 1998 season with the Mets. His impact was felt immediately. Piazza slashed .348, .417, .607 while hitting 23 HR and driving in 76 runs. Over his Mets career, he hit .296, 220, 655 with a .915 OPS in 972 games. Without Piazza, the Mets do not return to the postseason in 1999 and win the NL Pennant in 2000.
Going back to the original question that was posed, the answer probably depends on what direction the question is taken. For instance, Piazza may be considered the second greatest Mets player ever outside of Seaver. Out of the four players who were traded for, Piazza spent the longest time with the Mets after he was acquired. Clendenon performed his best in the 1969 World Series and provided a formidable bat that could compete with anything the Orioles could offer. Hernandez led the movement from the Mets being a loser to a winner.
The interesting consideration is Cespedes. The Mets offense needed a power bat and a leader (even if the leading was by example on the field by mere performance.) The Mets offense may need somebody to step up in the postseason and play his best in October. Cespedes, in his two postseason series, is a combined .350 (14-40) with 2 2B, 1 3B and 1 HR for the Athletics in 2012 and 2013. And with the announcement of the 5 day window being removed from Cespedes upcoming free agency, a chance exists that Cespedes can sign a long term deal with the New York Mets. So, a case can be made that Cespedes has a chance to be the best in-season trade acquisition in franchise history. Thoughts?