Mertens pushed Cahill in the back during the first half of Saturday's friendly at Wembley and caused a collision between the Chelsea man and goalkeeper Joe Hart.
The incident resulted in Cahill suffering a double fracture of the jaw and he now joins John Ruddy, Frank Lampard and Gareth Barry in being ruled out of the original squad and Liverpool's Martin Kelly has been called up as a replacement.
PSV Eindhoven player Mertens, who was yellow-carded for the challenge, said: "It was an accident. It was unintentional.
"He (Cahill) blocked me off and he hit his head (on Hart's shoulder). But there was no malice in what I did.
"Absolutely, I feel for him. If that was me in his place, getting injured like that a week before the Euros, I'd feel so bad.
"I'd feel terrible. I'm sorry he's not able to go."
Cahill's defensive team-mate Joleon Lescott - who now looks set to partner John Terry in the centre of England's back four - was unimpressed by Mertens' push and felt the Belgian could have been red-carded.
Lescott wrote on Twitter: "Gutted to hear Gaz is ruled out. The challenge that caused it was more reckless and dangerous than most 2 footed tackles so why only yellow."
Watch: Injured Cahill misses out on Euro 2012... More Videos
England skipper Steven Gerrard has revealed his anger - not at Mertens but at England losing so many key players in the build-up to the finals in Poland and Ukraine.
He said: "I am angry about it because we have lost some big players with a tournament eight or nine days away.
"That's the blow and that's the price you pay for these warm-up games.
"To do well in these tournaments you have to have your big players and to lose Gary is another huge blow on the back of Frank Lampard and Gareth Barry.
"His performances in the last six months have been colossal and he was a colossus in the Champions League final win. We needed him."
As regards Mertens' challenge, Gerrard said: "I don't think you can blame the guy because he was itching to score a goal.
"Maybe he got carried away but I don't think you can put any blame on the kid. But it is a massive blow for us."