"He's an exceptional player. The leadership he gives, the presence he has, and the effect he has on those around him, that's incredibly significant," Borthwick was quoted as saying by British media.
"The fundamentals of his game - scrum, his work in the tight, his mauling work, his line-out work, his set-piece work - are exceptional. And on top of that he is making 20-odd tackles a game and carrying the ball 15 times.
"His work-rate is absolutely phenomenal. It's a big loss and I'm really disappointed for him and disappointed for the team."
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Borthwick said the side have the strength in depth with Ellis Genge and Ben Moon set to compete for Vunipola's spot, although Genge was withdrawn from Thursday's training session with a sore ankle.
"Ben Moon and Ellis Genge have some real strengths as well. We have seen that before in an England shirt. I am looking forward to seeing how these guys take their opportunity."
England play second-placed Wales in Cardiff on Feb. 23 in a potential title decider and head coach Eddie Jones named 25 players for a three-day training camp in London earlier this week.
(Reporting by Rohith Nair in Bengaluru; editing by Sudipto Ganguly)
