Ronaldo, making his first Premier League start since August 13, managed just one shot before being subbed off in the 72nd minute in favor of Marcus Rashford.
The Portuguese superstar looked clearly annoyed as he left the pitch and continued to shake his head in the dugout as United struggled to break down Eddie Howe’s side.
United have Premier League games against Tottenham and Chelsea next week and Ten Hag said Ronaldo was withdrawn to keep him for those games.
“We have to recover because we have four games in 10 days and especially for the forwards, I want them to be fresh,” Ten Haag said.
“I want them all fresh, we have to save because of some problems.
“[Anthony] Martial is unavailable at the moment, then Rashy [Rashford] was bad, so he didn’t have the strength for the whole game.
“We had to plan to cover four games because we want to win or at least get a result in all four games.”
The result saw United record 76 clean sheets in the Premier League, 31 of which have come since Alex Ferguson’s retirement in 2013 (41 per cent) – the most by any team in the competition since the start of the 2013-14 season.
Despite the disappointing result, Ten Haag was pleased with his team’s performance and believes they deserved all three points.
“I’m disappointed with the result, but I’m happy with the game,” said the Dutchman.
“Many elements of the team, the manner of the game, we transferred to the field. We have seen it.
“Pressing throughout the game was good. Possession was better in the second half. We made an improvement after the break.
“Then you saw we pretty much dominated that game in the second half and also ultimately physically, again the mentality, how we kept going.
“You saw the challenges they had and they are one of the best physical teams in the league. Everyone had cramps and we didn’t because we let them run, because we had good possession and our pressing was good.”
The result left the Red Devils fifth in the Premier League, three points behind fourth-placed Chelsea and still one ahead of Newcastle.