A ruthless Leinster took their Champions Cup final frustrations out on the visiting Lions, running in nine tries in a 59-10 rout.
For the Lions, finishing the season on a negative note is never great, but van Rooyen chose to focus on the trajectory of his young Lions, who reached the URC playoffs for the first time this season.
"The undertone of this team is still very young," he told the press after the match.
"If you look at players like Batho [Hlekani], Hasheem [Pead] and Siba [Mahashe], there are a lot of players experiencing this level and this pressure for the first time.
"If you take tonight out of the picture, there is real excitement about this group, where we are and what is possible. But we also saw the next level tonight, and we aspire to get there.
"The camaraderie, the care the players have for each other and how hard they're willing to work for one another have all been major positives.
"Our alignment as coaches and players has improved. Our identity and how we want to play has become clearer. Our kicking game improved dramatically through the season and before tonight we were among the top attacking teams in the competition.
"It has been a season of growth, a season of firsts and a season that showed what this group is capable of.
"We'll come back with plans for growth and progression and keep building."
Another measurement of success for the Johannesburg franchise is how many individuals are getting a look-in at the national level. Across the two Springbok alignment camps this year, no less than nine Lions players were invited.
"Two or three seasons ago we only had one or two players in the alignment camp and now we've got eight or nine," continued van Rooyen.
"It's fantastic for the players and for the union. The experience they bring back makes the whole group stronger.
"It does create different challenges because you have to manage workloads and opportunities, but those are good problems to have."