"I wanted to go out and get the win, but I didn't expect to play as well as I did," said Cruz, a 6-foot, 185-pound junior. "I guess I exceeded expectations. To be honest, I completely shocked myself."
Roman coach Joe McCourt said Keir's playing status was a game-time decision. Cruz had practiced most of the week with the first-team offense.
"I told Chris, 'We don't need you to win the game, but we need you to manage it,' " McCourt said. "Instead, he went out and took over the game."
Trying for their first league title since 2007, the Cahillites will face three-time defending champ La Salle in the league final at 7 p.m. Friday at Northeast.
Cruz, also an outside linebacker, connected on 8 of 14 passes. In the third quarter, he ran for a 62-yard score, neatly breaking a tackle near midfield, to give Roman a 31-17 advantage.
"I think I have a pretty good arm, but I'm probably a bigger running threat," said the 17-year-old, who carried 13 times for 90 yards.
The turning point came just before halftime, with the Hawks ahead by 17-7 and fresh off a successful goal-line stand.
Roman tackle Tyrone Brown surged into the backfield and corralled tailback Joe Lavalle in the end zone for a safety. Six plays later, Cruz hit Taishan Tucker across the middle for a 24-yard TD, and followed it, with a defender draped on him, with a 2-point conversion toss to Marcus Kelly.
"You always play harder when you're losing," Brown said. "You're motivated to make a comeback and get the win. It's go hard or go home."
Cruz and Tucker (five catches for 147 yards) combined for a third score, a 17-yarder, to make it 24-17. Athletic wideout William Fuller was on the receiving end of Cruz's fourth and fifth TD passes.
Roman, reversing a 22-point loss to the Prep two weeks ago, totaled 482 yards on offense. Kelly carried 22 times for 126 yards.
A defense that consistently pressured Prep QB Skyler Mornhinweg (15-for-32 passing, 270 yards) was anchored by Brown, end Jack Schanz, and linebackers Kenneth English (11/2 sacks, two hurries) and Brett Pellicciotti.
Roman Catholic 7 10 21 7 – 45
St. Joseph's Prep 0 17 0 0 – 17
RC: Tucker 53 pass from Cruz (Isaac kick)
SJP: Lavalle 1 run (Battin kick)
SJP: Medes 43 pass from Mornhinweg (Battin kick)
SJP: FG Battin 38
RC: Safety
RC: Tucker 24 pass from Cruz (M. Kelly pass from Cruz)
RC: Tucker 17 pass from Cruz (Isaac kick)
RC: Cruz 62 run (Isaac kick)
RC: Fuller 18 pass from Cruz (Isaac kick)
RC: Fuller 53 pass from Cruz (Isaac kick)
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.