Johansen struggled but finished the game, which Nashville lost 3-2 in overtime, before encountering difficulty in the locker room.
"It happened pretty fast," Johansen said. "I got off the ice and I was having a tough time just standing and getting my gear off, and then obviously I went right back to the medical guys on our team. First thing, looked at it, looks like it was going to be a charley horse, started swelling up, so started icing it and doing all that stuff, and then I was going to go shower and come back and put some wrap on it, and then get home and get some rest.
"By the time I walked to the shower, finished showering, and tried to get on my underwear, I couldn't walk. I had to get one of the guys to grab a trainer and come back and help me to the medical room. And then the docs looked at it again, and the swelling increased by like triple it seemed in those 20 minutes when I was walking around and getting ready to go home.
"It turned into pretty quick transition where I just needed to head over [to the hospital] and get it looked at and take the next step, I guess."
The Predators said Johansen was treated for an acute compartment syndrome of the left thigh, an injury that can cause nerve and muscle damage because of pressure decreasing blood supply.
Johansen said Wednesday he is expected to make a full recovery.
The 24-year-old was unable to play in Games 5 and 6 against the Ducks, which the Predators won to reach the Stanley Cup Final for the first time in their history. He finished the Stanley Cup Playoffs with 13 points (three goals, 10 assists), second on the Predators to linemate Filip Forsberg (15 points; eight goals, seven assists).
"I just had a lot of confidence in my ability at the time," Johansen said. "Obviously a big reason why it [stinks] so much right now that I can't play anymore."
Games 5 and 6 of the conference final were the first games Johansen missed since he was traded to the Predators from the Columbus Blue Jackets during last season. His streak was 152 in a row, regular season and postseason, according to The Tennessean.
Game 1 of the Stanley Cup Final is Monday at either the Ottawa Senators or Pittsburgh Penguins. They will decide the Eastern Conference Final in Game 7 at Pittsburgh on Thursday (8 p.m. ET; NBCSN, CBC, TVA Sports).
"It was pretty killer," said Johansen, who was on crutches on the ice for the Western Conference trophy celebration. "It's definitely been making it a lot easier right now, the guys being successful and putting themselves in a position to win the Cup.
"It's been pretty tough, I won't lie, it's been difficult and I'm sure it's going to be a little frustrating not being out there even moving forward now."