If the Bulls were doubting Thomases before Sunday afternoon, Isaiah made them pay for it dearly with each turn of his hand, each herky-jerky dribble and each devastating floater.
Perhaps wiser to their errors, the Bulls will go into Game 5 in Boston with an Isaiah of their own in the starting lineup, as Isaiah Canaan will make the start at TD Garden on Wednesday night.
Having been banished to the inactive list and end of the bench, Canaan's best moments under the United Center lights had been one-on-one matchups with his teammates hours before the real games began.
So being tossed in with 2:15 left in the first quarter and the Bulls already behind 13 points presented both opportunity and some nerves given the stakes. Having played a total of 92 minutes since the calendar turned to 2017, beggars can't be choosers.
"Throughout the season you just wonder when that opportunity is going to come," Canaan said. "I believe everything happens for a reason. You never can see what it may be, from going from not playing for the second half of the season mostly to playing in the most crucial time of the season in the playoffs, I guess God just sent me a sign to mentally be locked in and really battle your faith."
He responded by pressuring Thomas in a way Jerian Grant and Michael Carter-Williams couldn't with any effectiveness, probably due to his quickness and similar build to Thomas.
"My job is to make his job miserable," Canaan said. "He's a great player and he can find different cracks, but I'm just going out there trying to interrupt their timing, try and milk as much time as I can off the shot clock and just keep him uncomfortable. Not let him get settled in for what they want to run, so I'm just out there to make him uncomfortable and speed him up."
Canaan scored 13 points in 33 minutes, hitting three 3-pointers and being a +11 while on the floor, so it's easy to see why Bulls coach Fred Hoiberg wasn't going to mess around with the Grant and Carter-Williams combo that hadn't done much of anything in the friendly confines of the United Center over the weekend.
"I really thought he did a good job picking up the ball and pressuring the pickup point a full 94 feet," Hoiberg said. "I thought his initial ball pressure was good. We have to get off to a much better start if we want to have any chance of winning another game in this series. That's two games in a row now we've gotten ourselves down 20 points and fought all the way back."
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Still, Thomas scored 33 points and exploited a once-aggressive Bulls defense that seemed to be on its heels for most of the second half against Thomas, leading to Hoiberg's claims about Thomas carrying the ball to give himself an advantage in his hesitation moves.
Hoiberg wouldn't revisit the topic before Monday's practice and praised the Celtics guard who's battling through the devastating loss of his sister, who died right before the series opened in Boston last weekend.
"I'm a huge Isaiah Thomas fan. He's as big a competitor as we have in our league," Hoiberg said. "Obviously, he's as tough a guy to guard as we have in our league. I think the world of the kid, especially what he's going through now, how he's fought through that and like I said last night, he's a warrior to go out there and do what he's done so far in this series."
Thomas has averaged 25.8 points on 45.8 percent shooting with six assists and four rebounds per game. At times, the Bulls' size has bothered Thomas at the rim, particularly Jimmy Butler coming over to help alter his share of attempts, but since the Celtics have altered their approach Thomas has found easier traction to the basket.
Celtics coach Brad Stevens inserted Gerald Green in the starting lineup and the move has worked masterfully, with Green scoring 18 points with four 3-pointers and one highlight-worthy baseline dunk.
It's rendered Robin Lopez effectiveness to a minimum, as he was an unsung hero on the offensive glass in the first two games, meaning Stevens' move to play smaller was a risk, albeit a calculated one.
The Bulls didn't adjust at all to it in Game 4 after the Celtics won decisively Friday night, but changes besides Canaan have to be in store if the Bulls hope to come home looking to clinch a berth in the second round as opposed to fighting off elimination in Game 6.
"It changed it entirely. Obviously when it was big-big, we dominated them," Dwyane Wade said. "Dominated them defensively, incredibly. They made adjustments, we didn't need to because we were up 2-0. Then coming off Game 3, we felt watching the film we played with low motor, not enough energy so we didn't feel the gameplan was the key.
"But it was coming out of Game 4. So now it's a few adjustments we hope to make and hopefully it makes a difference."