Mitch Trubisky threw for 166 yards and led three scoring drives after Mike Glennon struggled, before the Denver Broncos scored two long touchdowns in the closing minutes to beat the Bears 24-17 in the preseason opener on Thursday night.Though the Bears have insisted Glennon is their starter, Trubisky just might have created a quarterback controversy.Denver’s Trevor Siemian,a former Northwestern Wildcat, did nothing to hurt his standing as the leader for the starting quarterback job, while Paxton Lynch did little to close any ground.Third-stringer Kyle Sloter threw a 47-yard touchdown pass to Isaiah McKenzie with about six minutes left in the game, and De’Angelo Henderson ran for a 41-yard score with just under two minutes remaining to put the Broncos on top 24-17.But the biggest development in this game was Trubisky showing exactly why general manager Ryan Pace traded up a spot with San Francisco to draft him with the No. 2 overall pick.The North Carolina product completed 18 of 25 passes in his first pro appearance after making just 13 college starts.Trubisky replaced veteran Mark Sanchez late in the second quarter and completed his first 10 passes while leading the Bears to touchdowns on his first two possessions and a field goal on the next one. That gave the Bears a 17-10 lead.Glennon had a rough time in his first appearance for the Bears. Signed to a three-year contract after the Bears parted with Jay Cutler, he was 2 of 8 for 20 yards with a zero QB rating. Chris Harris Jr. intercepted an overthrown pass on the game’s third play from scrimmage and returned it 50 yards for a touchdown.Things didn’t get much better for Bear’s offense until Trubisky came in with 1:55 left in the half.He completed four passes on a 50-yard drive, connecting with Victor Cruz from the 2 to cut it to 10-7 with 13 seconds left.Trubisky then led the Bears on a 75-yard drive early in the third, going 5 for 5 for 50 yards before Benny Cunningham ran it in from the 1 to give the Bears a 14-10 lead.Siemian – a surprise successor to Peyton Manning last season – completed 6 of 7 passes for 51 yards and led a field-goal drive.Lynch came on early in the second quarter and was 6 for 9 for 42 yards.