by John Jackson, The Chicago Sun-Times , Chicago Sun Times

Thomas, who was sent home two days earlier in the week because of the flu, was scheduled to undergo surgery today and is expected to be sidelined from four to six weeks.

''We are disappointed to lose Tyrus,'' general manager Gar Forman said. ''This will be an opportunity for Taj [Gibson] and James [Johnson] to get valuable experience.''

Power forward is one of the deeper positions on the team. Besides the two rookies, Brad Miller, Joakim Noah and even small forward Luol Deng can move over to play minutes at the position.

Still, the bulk of the minutes likely will go to Gibson, who has started the last two games because of Thomas' illness and will start tonight when the Bulls host the Charlotte Bobcats at the United Center.

Johnson (No. 16) was drafted ahead of Gibson (No. 26), but Gibson showed from Day 1 of training camp that he was NBA-ready. He has defensive instincts that few rookies possess right from the start.

''[Gibson has] been great,'' Bulls coach Vinny Del Negro said. ''Taj keeps working, he's out there after practice working; he's constant effort. He's still making some mistakes, but you can live with those because he's always working and his energy and his effort -- especially defensively -- is good.

''He just has to continually find his spots in the offense, but as he gets more confident and gets more consistent minutes, that will come.''

Gibson had a good game Thursday in the Bulls' upset of the Cleveland Cavaliers, with 11 points and seven rebounds. Still, the big beneficiary of the Thomas injury is Johnson, whose only playing time in the first five games was garbage-time minutes at the end of the blowout loss at Boston last week.

The 6-9 former Wake Forest star has more raw talent, but struggled in the preseason to harness that ability. Johnson also has the disadvantage of having to learn two positions, power forward and small forward.

''[Johnson has] been playing a lot better, practicing a lot better the last couple of weeks, which is good for him,'' Del Negro said. ''If he gets out there, I just expect him to play hard.''

Whether Johnson steps into the rotation remains to be seen. In the Bulls' win over the Milwaukee Bucks on Tuesday -- which Thomas missed -- Del Negro opted to go with eight players. Miller, Kirk Hinrich and Jannero Pargo were the only reserves to see action.

But with Thomas out for at least a month -- and with a grueling five-game West Coast trip looming this month -- Del Negro may be forced to lengthen the rotation.

Thomas has been inconsistent throughout his time with the Bulls , but the team was looking for him to make a major step this season.

However, he got off to a somewhat sluggish start and Del Negro opted to go with other players during the fourth quarter of games the first week of the season. Thomas, who was averaging 8.3 points and 5.3 rebounds, wasn't happy about that and also was upset he was called out by Del Negro during a film session for a slow defensive rotation on Monday.

The next day Thomas was sick. He traveled on his own to join the team in Cleveland just hours before the game after getting clearance by a team doctor.

The Bulls provided few details of how the injury occurred, saying only that he was lifting weights during practice, which is not unusual.

''He was doing some type of pull-up or something and kind of injured his arm,'' Del Negro said. ''It's just unfortunate.''

TONIGHT

Bobcats AT Bulls

The facts: 7, Ch. 9, 1000-AM.

The story line: The Bulls will be aiming to win their third straight game and remain unbeaten at the United Center. The Bobcats went into Friday night as the league's lowest scoring team (79.8 points per game, 36 percent shooting), but then they rolled past the visiting Atlanta Hawks 103-83.

John Jackson

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