Alonso made nine changes to the side that lost 2-1 to Arsenal in their previous fixture for the League Cup clash with Leeds.
Cheslea went into half-time 1-0 down to the West Yorkshire club and had been bullied for much of the first 45 minutes.
The Spaniard made the call to bring on Cole Palmer, Morgan Rogers, Reece James, and Pedro Neto at the break.
A 53rd minute penalty with the score at 2-0 shifted momentum in Chelsea’s favour, and the home side were eventual 6-3 winners.
Speaking to Sky Sports after the game, Alonso took full responsibility for a disappointing first half performance.
"Happy with the result and the reaction," he said.
On team selection: "Yeah, there was so many changes. There was a risk. Things weren’t going the way we planned, so we had to change. I take responsibility with the first half. The crowd were great, with the atmosphere (pushing the players on)."
On impact of subs: "I like that they were ready before the game. They knew they weren’t starting and they were ready to make an impact.
"When you put together a team that hardly plays together, it is difficult to play at this level when (Leeds United) are intense."
On Morgan Rogers and Cole Palmer's chemistry: "For sure, it’s so important. To have a great chemistry between them. They enjoy assisting together. It’s important for the team."