Having missed four games with a thigh injury, Ronaldo was back in the starting line-up and back among the goals, heading in the opener in the 45th minute for his eighth goal in his last five Champions League matches against Ajax.
The scoreline was harsh on Ajax, but they were level straight from the restart.
"Ronaldo showed that he is on a different level".
Ronaldo, who was playing for the first time since scoring a hat-trick against Atletico Madrid in last month's round of 16, celebrated his goal in front of Ajax fans and appeared to have a beer cup thrown in his direction from the crowd.
"Ajax have great qualities".
"We suffered after their goal because we were a bit shocked to concede immediately after the break but we defended well and finished the game better".
He also hinted it would be a 4-3-3 formation, or even 4-4-2, because "there are only three centre-backs available, so I doubt I'd use all of them".
"We can use this game as an example, we can learn from it".
Ten Hag admitted that it "isn't exactly what we were hoping for, but it is still a good result".
His brilliant young side is likely to be broken up at the end of this campaign, so this may be the Amsterdam club's best chance in a long time to win a fifth European Cup.
Barcelona-bound star Frenkie de Jong produced another impressive performance for Ajax and the 21-year-old midfielder believes the Dutch giant can progress to the semi-finals.
Brazil global David Neres scored on the other side of the break with a superb curling effort.
The Juventus boss praised the player and said he was the real source of the player.
Allegri also praised Ronaldo, who had missed the previous four games with a thigh problem picked up on global duty.
Playing for the first time since suffering a muscle injury on global duty last month, Ronaldo broke the deadlock via a diving header on the stroke of half-time.
The 34-year-old forward scored his side's only goal in the 45th minute of the meeting, but David Neres 46th minute equaliser cancelled out the Portuguese's first half goal.