Coming off the bench, usual starters Nafis Walker, Toyon Wynder, and Jarrell Haywood combined for 35 points.
"I got into some trouble, and Coach Johnson did what he had to do," Walker said. "I respect any decision he has to make."
Walker, a 6-foot-3, 185-pound wing guard, delivered a game-high 20 points, including four three-pointers, six rebounds, and two assists.
"I was upset with myself, that I possibly could have let down my team with my actions," the 17-year-old said. "That's the last thing I wanted to do."
Wynder, the third player to come off the bench, hit two three-pointers on the way to seven points.
Haywood, a long-range sniper who did not enter the game until the third quarter, managed eight points, including a pair of three-pointers.
On Tuesday, the Panthers will visit No. 4 seed Central Bucks West, a 46-36 winner Friday night over No. 29 West Chester East.
After getting the starting nod, guards Aaron McGlawn (eight points, six assists) and Nile Lyons (five first-quarter points) helped Cheltenham (15-8) build an 18-11 lead.
"We've been in this situation a few times before, so those guys are ready," said Johnson, a math teacher at the school. "There's no nervousness or fear."
Against their Suburban One League American Conference rivals, the Panthers shot 7 for 16 from beyond the arc in the first half. Overall, they were 11 for 26.
Guard John Decamara and 6-3 forward Andrew Derr each posted 16 points for Upper Dublin (13-10). Decamara, who came in with 77 treys this season, was 4 for 7 from beyond the arc. Fellow senior Jared Zarwin dished off eight assists.
Derr's layup, off a nifty feed from Zarwin, closed the gap to 51-50 with 1 minute, 23 seconds to play. From there, the hosts converted 5 of 6 free throws.
Upper Dublin 13 13 13 11 – 50
Cheltenham 18 13 16 11 – 58
UD: John Decamara 16, Jared Zarwin 1, Darrell Harrison 4, Andrew Derr 16, Andrew Carber 13.
C: Nafis Walker 20, Toyon Wynder 7, Nile Lyons 5, Dillon Tolliver 6, Aaron McGlawn 8, Guy Henderson 4, Jarrell Haywood 8.
Contact staff writer Rick O'Brien at 610-313-8019, robrien@phillynews.com,
or @ozoneinq on Twitter.
Read his blog, "The O'Zone,"
at www.philly.com/ozone