ESPN has a really interesting read on some behind-the-scenes scrambling at the MLB trade deadline as told by GMs:
https://www.espn.com/mlb/story/_/id/45661110/mlb-trade-deadline-2025-gms-best-stories-deals-fell-rumors
The second-to-last story is from Zack Minasian during his time with the Brewers, when the Brewers almost had a deal for Felix Hernandez:
"Jack (Zduriencik) and Doug (Melvin) were talking about a trade that would have sent Felix Hernandez to Milwaukee," Minasian recalled. "At one point, we thought Jack had agreed to it but he needed to make one other move before we could finalize it.
"It didn't happen, but for an hour we thought we were getting Felix Hernandez. We were nervous, anxious, excited and just waiting."
It doesn't say what year that was, but looking back you can probably isolate it to 2011. It would have been very unlikely that SEA would have been willing to trade Felix before 2011, especially since that was his age 25 season and he won the Cy Young in 2010. That, and the Brewers wouldn't have been sellers during that time to acquire Felix as a prospect. Seattle wouldn't have been buyers during that time either. While it may seem unlikely that the Brewers could acquire him in 2011, in that it might have literally cost the farm, it wouldn't be unprecedented since the Brewers went all-in for Sabathia in 2008.
It would be unlikely that the Brewers would have traded for him between 2012 and 2015 as they finished 3rd in the division twice and 4th twice during that stretch, so no need to make a big acquisition. Melvin then moved out of the GM role in August of 2015 and Zduriencik was fired two weeks later.
What could have been to have Felix Hernandez in the 2011 playoffs...