Even the Arkansas freshman didn't know just how good.
Led by Portis' career-high 35 points, the Razorbacks rallied late and held on for a 65-58 win over Alabama.
They did so thanks to a career night from their McDonald's All-American, who set the school record for most points in a game by a freshman. Portis passed Scotty Thurman's mark of 34 points, which was reached twice during the 1992-93 season, and he did so with Thurman watching from the bench.
By the end of the game, which Arkansas (14-8, 3-6 Southeastern Conference) needed badly after losing two of its last three at home, chants of "Bobby, Bobby" could be heard throughout Bud Walton Arena.
Portis started his night with a pair of quick jumpers, scoring Arkansas' first eight points. The hot start was a sign of things to come.
"I hit those first two jumpers, and then I just knew, 'Oh yeah, I'm fixing to have a big night now,'" Portis said.
Portis added nine rebounds and a career-best six blocks in the best effort of his young career, finishing 14 of 17 from the field on a night when the rest of his teammates needed someone to carry them.
The rest of the Razorbacks combined to finish 8 of 40 (20 percent) from the field, but the lackluster effort was enough to earn the win — thanks to Portis. The freshman topped his previous career-best of 19 points, which came in a 104-71 win over Texas San Antonio on Jan. 4.
"He had it going on," Arkansas coach Mike Anderson said. "It seemed like when he touched it, good things took place."
Rodney Cooper led the Crimson Tide (9-13, 3-6), which has now lost five of its last six games, with 22 points, while Trevor Releford added 12.
Alabama shot just 21 of 59 (35.6 percent) from the field, including 4 of 17 on 3-pointers.
The Crimson Tide led 26-24 at halftime following a buzzer-beating jumper by Cooper to end the first half.
Retin Obasohan extended Alabama's lead to 30-24 early in the second half with a layup, but Portis followed by continuing his hot play of the first half.
The freshman scored Arkansas' first 11 points of the second half, making it 17 straight points overall, to put the Razorbacks up 35-33.
"Bobby Portis, tonight, was the story of the game," Alabama coach Anthony Grant said. "For a freshman, to able to do what he did tonight ... He was outstanding."
Portis had 29 of Arkansas' first 35 points, and he was 12-of-15 shooting at that point — compared to just 1 of 25 for the rest of his teammates. He passed the previous mark of Thurman — now a member of the Arkansas support staff — with a pair of free throws with 18.6 seconds remaining, giving the Razorbacks a 63-58 lead and putting the game out of reach.
"(Thurman) will be pretty mad about it, I know he will," Portis joked.
Alabama's last lead came on a pair of free throw by Cooper to go up 46-45.
Alandise Harris, who was suspended for Arkansas' last game at LSU, followed with a jumper to put the Razorbacks up for good. The points capped a run of eight straight points for the junior, who scored all of his points during the stretch.
Both teams entered having lost four of their last five games, with the Crimson Tide's losses coming by an average of 15 points.
Alabama still hasn't won away from home in nearly a year. Its last road win was a 52-45 win at Georgia, on Feb. 12, 2013, and it's 0-12 away from home since then.
"Their defense limited us offensively in terms of what we were able to do," Grant said. "We weren't able to have much success offensively."
The Crimson Tide shot just 33.3 percent (9 of 27) in the first half, but Arkansas was worse — connecting on just 9 of 33 shots (27.3 percent).
Portis had eight of the nine first-half field goals for the Razorbacks, and his six blocks topped his previous best of three.
"We needed him," Arkansas senior Mardracus Wade said. "He stepped up big for us, just like we always thought he could ... I'm not surprised at all by what he did today."