I’m sure there are many such, but I was just reading about Tony Conigliaro’s beaning in 1967 and thought “Gee, that should have devastated the Red Sox,” who of course went on to win the 1967 AL pennant. Either through injury or retirement (such as that of Stan Musial, who retired after 1963, but the Cardinals went on to win the World’s Championship in 1964), who do you think is the largest loss endured by a team that went on to win a pennant?
It would be hard to top Cleveland in 1920, who saw the great Ray Chapman literally KILLED by a pitch, and then went on to win the World Series.
Cleveland, luckily, had a kid named Joe Sewell backing up Chapman, and Sewell was a hell of a player himself.
'75 Red Sox won without Jim Rice.
1939 Yankees lost their Captain to Lou Gehrig’s disease. They won the pennant despite this and swept the Reds in the World Series.
To make a game of this question, we could rank these losses. For example, of the five players mentioned, I’d say the most devastating loss that resulted in a team triumph would be Gehrig and the least devastating would be Rice. I’d award points for
- the team’s final showing: i.e., if a team won the World’s Series in four games, that’s a good showing, and if it “only” won a league pennant with a low number of victories, not so much.
- the player’s quality: i.e.losing a player who was clearly the team’s best player counts much more than losing a merely very good player.
- how much the player was able to contribute in the season he was lost. I.e., Jim Rice was lost very late in the regular season and made major contributions to the 1975 Pennant, and Musial simply didn’t play at all in 1964, so Musial scores high and Rice scores low.
I’d also add belatedly a Free Agent leaving the club would be another kind of loss.
So my rankings so far are:
- Gehrig
- Musial
- Chapman
- Conigliaro
- Rice
You can play along with this scheme (or argue with it), or just nominate players who missed time on pennant-winning teams.
Adding to Gehrig on the other side of the ledger, the 1939 Yanks ended up being one of the greatest teams ever. They finished with over a .700 win pct & 17 games ahead of 2nd place Red Sox. A lot of players stepped up as Babe Dahlgren 1939 vs Lou Gehrig 1938 was a big drop. By WAR, Babe was -.6 and Gehrig '38 was a 4.7.
Yeah, but the Yankees that year were really strong, finished 17 games up of the Red Sox. They didn’t need Gehrig. As for psychological effects on his teammates of losing him, they went 16-3 after Lou played his last game.
Rice played 144 games so contributed heavily to the team’s success. They were well on their way to win the East when he played his last game that year.
I’m going in a different direction. I often think, why did the Orioles hold on to Cal Ripken? The economics were a bit different back in 1994 or so. He had his best year in 1991 but his next two were below league average, bounced back a bit in 94 and was really below average for the rest of his career except for 1999 at age 38. Maybe a smaller market team would have traded him at the deadline for some value, say in 1995. They did win the East in 1997 but just four games out in 1996 but weren’t good for a while after that. Maybe replacing him would have made things better in Baltimore in those years. Yes, he was popular and revered in Baltimore and fans at first would have been furious. But many such players got traded for the betterment of the team before and since, it’s actually not expected such a player to play his whole career with one team anymore.
1979 Reds. Pete Rose signed with the Phillies before that season. The Reds win the West, the Phillies are 14 games back in the East. Everyone in Cincinnati was mad and didn’t think the Reds had any chance to be competitive by losing this one player.
I was going to ask, “Who was their captain?” But that joke is too obvious. 
Came up with a few more: Kaline went down to an injury in 1968, but the Tigers won the championship with him on the bench (though he did recover in time to play in the Series, causing a pileup of four excellent outfielders, one whom had to play the infield in the Series) and Bob Gibson was injured the previous season (broken leg), again missing some crucial games but recovering in time to pitch well in the Series.
Some comic had a bit about “What were the odds that Lou Gehrig would suffer from Lou Gehrig’s disease?”
My 2019 Nationals turned out to be a great case of addition by subtraction. Bryce Harper signed with the Phillies in the off-season but we won the World Series anyway.
In 2011 the Cardinals won the World Series with Albert Pujols, who then signed with the Angels. In 2012 the Cardinals barely grabbed a Wild Card spot, but then beat the Nationals in the first playoff round before losing the Championship Series to the Giants four games to three.
Probably not at the same level of some of the others, but the 1982 Brewers won the AL pennant despite losing one of their biggest stars, closer Rollie Fingers, to a torn muscle in his arm at the beginning of September. At the time, Fingers was the reigning AL MVP, Cy Young winner, and Reliever of the Year.
Not having Fingers available in the bullpen was undoubtedly a factor in the Brewers losing the World Series to the Cardinals, in seven games.
The 2001 Seattle Mariners didn’t win the AL pennant but do merit an honorable mention since they set a record with 116 victories even after losing Ken Griffey Jr and A-Rod.
Also, although it’s the NHL, the 1989-90 Edmonton Oilers still had enough talent to win the Stanley Cup even after losing the greatest hockey player of all time, Wayne Gretsky, in 1988.
Was just going to mention them. Griffey left after the '99 season and A-Rod after 2000. They lost Randy Johnson, Griffey, Jr, and A-Rod in that timeframe but were still competitive.
Well, that year, but not for more than 2 decades after…
(They’re finally decent again, but it took awhile.)
They still had Edgar and they added Ichiro.
It’s interesting to look at teams that lost stars but were still competitive, and at what point they lost it. I was thinking of the Oakland A’s, who turned into a pumpkin at the start of the free agent ball, but it didn’t happen at the stroke of midnight. They lost Catfish Hunter in 1975, but still won the division with a 98-64 mark. They lost Reggie Jackson and Ken Holtzman in 1976 but still turned in a respectable 87-74 second-place finish.
Finally, after Joe Rudi, Sal Bando, Bert Campaneris, Rollie Fingers, Phil Garner, and Claudell Washington (and others) departed, they finished in last place in 1977. A team can lose one or two great players, as long as they have solid players at other positions. It’s more a matter of having an average player to plug the gaps.
This happened a few times with the old Philadelphia A’s, when Connie Mack would sell off his stars under economic pressure. Each time he thought he had kept enough good players around, but they always tipped over into losing.