Complex sentence: The Brazilian's arrival reversed the fortunes of not only the declining Cosmos, but the perception of the entire league; with the credibility that Pelé gave it, soccer became a viable alternative to "American" sports such as basketball, baseball and American football.
Split into simpler sentences: The Brazilian's arrival completely reversed the fortunes of not only the declining Cosmos, but the perception of the entire league. With the credibility that Pelé gave it, soccer became a viable alternative to "American" sports such as basketball, baseball and American football.