The center left in the second period of a 5-3 win against the Florida Panthers at T-Mobile Arena in Las Vegas on March 17 with what was considered an upper-body injury.
"What most people probably wouldn't know is he broke his thumb in a bad way about three or four weeks into his return," McPhee said on "The Bob McCown Podcast." "It was non-displaced, so he didn't have to have surgery, but it was a bad fracture, and the guy didn't miss a game.
"He toughed it out. We had to numb him up and it wasn't easy because when that wore off he was in a lot of pain. But he toughed it out for six weeks with that thumb."
Eichel had 25 points (14 goals, 11 assists) in 34 games after he was acquired in a trade with the Buffalo Sabres on Nov. 4. He had artificial disk replacement surgery seven days later and made his season debut Feb. 16.
"We did as much research as you could do and, boy, there weren't many second thoughts," McPhee said. "He got drilled a few times this year, no issues whatsoever.
"I think he is what we thought he was. He is a very genetically gifted guy, a very talented guy. He really loves the game, I guess I didn't know enough about that."
The Golden Knights were ravaged by injuries this season. They played 46 games without forward Mark Stone and 43 without forward Max Pacioretty. Forward Reilly Smith did not play the final 24 games. Defenseman Alec Martinez missed 53 games because of a facial injury sustained Nov. 11.
Goalie Robin Lehner missed the final four games of the regular season and had shoulder surgery May 4. McPhee said the Golden Knights thought it was something he may have been able to play through until he got to where he could not. Lehner is expected to be ready for the start of next season.
"The injuries, they're not an excuse. It was a fact this year," McPhee said. "It was just a fact that seemed to wreck everything we tried to do."
Vegas finished 43-31-8, three points behind the Nashville Predators for the second wild card into the Stanley Cup Playoffs from the Western Conference and did not qualify for the first time since entering the NHL at the start of the 2017-18 season.
"It keeps you humble," McPhee said. "It's a good league and if you don't perform your best at all times, this is what can happen. It may have been our best team. We'll never know now."
Peter DeBoer said May 3 that he hoped to return as Golden Knights coach. General manager Kelly McCrimmon said DeBoer is under contract and they were scheduled to talk later that week. Even with Vegas coming off a non-playoff season, McPhee said there's been no discussion about starting from scratch.
"I don't think I'd ever be inclined to do that," McPhee said. "It's not easy to move contracts and so on.
"We're not finished with our analysis of this last season, There's lots of things you could go through, what could we have done better and things we'll just never know about because we never had a chance to do it."
NHL.com independent correspondent Paul Delos Santos contributed to this report
