Playing extended minutes at power forward with Bosh sidelined by an abdominal strain took its toll on the league MVP in Tuesday's 78-75 loss to the Indiana Pacers.
"It's a lot more taxing being in there with bigger guys," James said, according to the Miami Herald.
He said that defense "is the biggest difference. When you're on the perimeter, there's more space. The interior is more cramped and physical."
The stats say that the Heat lost the interior battle as Indiana tied the second-round playoff series 1-1. The Pacers outrebounded the Heat 50-40 after Miami took that stat 45-38 in Game 1.
Points in the paint were even at 38 in Game 2, but many of Miami's interior points came from drives by James and Dwyane Wade. The Heat dominated that stat in Game 1, 52-40. Bosh had 13 points and five rebounds in only 16 minutes before going down.
While James scored 28 points and Wade added 24 Tuesday, no other Heat player scored more than five points. Big men Ronny Turiaf, Joel Anthony and Udonis Haslem combined for seven points and 12 rebounds. Pacers forward David West had 16 points and 10 rebounds, while center Roy Hibbert had eight points and 11 rebounds.
Bosh also missed nine games during the regular season, and the Heat went 4-5. With Bosh on the court, Miami has the third top-scoring frontcourt in the league. Not only did he average 18 points and 7.9 rebounds per game, he also was the Heat's most clutch fourth-quarter shooter.
Since Bosh has been ruled out indefinitely, the Heat will have to make adjustments for Thursday's Game 3 and beyond. James has acknowledged it's going to be tough, "but I'm ready for the challenge," he said, according to the Herald.
Saying that he has played more different positions this season than he ever did in Cleveland, James also said that there are advantages to playing power forward.
"I can get more rebounds and start the break," he said, according to the Herald. The box score reveals, though, that the Pacers had 13 fast-break points to six for the Heat Tuesday.
Bosh's absence not only means more physical play for James, it also means that he's getting less rest. He has played 43 of 48 minutes in both games of the series. It could be argued that fatigue contributed to James missing two free throws with the Heat down one and 54 seconds left Tuesday.
"Forty minutes in the playoffs is different than 40 minutes in the regular season," James said, according to the newspaper. "Intensity is raised. The grind is much more intense. Hopefully, I can get a few minutes here and there."
Heat coach Erik Spoelstra doesn't know how much he can do about it.
"There's nothing we can do [about rest for James] in the fourth quarter," Spoelstra said, according to the Herald.
"Forty-minutes-plus is probably what he can expect."