The Teessiders entertain Championship leaders Cardiff on Saturday having played seven games inside 26 days in February, the last of them being Wednesday night's 2-0 FA Cup fifth-round defeat by European champions Chelsea.
Mowbray said: "In the Championship it is relentless. We play every three days in the Championship for 10 months, it seems.
"You ask the same players to go and do it again every single week, which is why, particularly in the Championship, you pick up huge amounts of injuries, which we have had.
"Professional footballers, when they are winning games and they are doing well, they just want to play.
"Instinct takes over as a footballer. As a boy, you grew up playing practically every night, a game of football, so when you become a professional, it's not such a burden to play, play, play."