Richarlison put the hosts in front in stoppage time but Matthijs de Ligt headed home a dramatic equaliser in the 96th minute.
There had also been boos at half-time, but Frank defended his decision to bring Simons off.
"Yeah sometimes you're lucky," Frank said of the impact from his substitutions.
"But of course every single time I take a decision to sub a player it’s with one aim. To make the team win or get back into the game or try to see the game out. It’s with only one aim. To try to win the game.
"Mathys did well, great goal, great involvement and in general I think Xavi did a fine game. A positive but also you need a bit of freshness."
Xavi Simons started for Tottenham in the No10 role against Manchester United
Pushed on whether he heard the boos after Simons' withdrawal, Frank added: "I'd just say every single time I make a decision, there's one aim and that is to make the team better."
There was also a blunt response when Frank was asked what he wanted from Tel when replacing Simons.