The 29-year-old has made the move to Old Trafford in a £40million deal to team up again with United boss and former Blues chief, Jose Mourinho.
Chelsea have signed Tiemoue Bakayoko as a replacement but Cahill is concerned how big an impact Matic's exit will have on their squad.
The Serbia international, who rejoined Chelsea from Benfica in 2014, made 154 appearances in all competitions for the Blues, scoring seven goals and managing 20 assists as he usually performed a fine defensive midfield role at Stamford Bridge.
Cahill told Sky Sports: "For me, Matic is a fantastic player. I had the benefit of him playing just in front of me and I realised how good he is on and off the ball.
"He'll be a huge loss."
Matic began United's UEFA Super Cup defeat against Real Madrid on Tuesday night.