Mullen faces a maximum four-year ban from rugby league after returning a positive sample to Drostanolone late last year.
Houston played alongside the halfback in Newcastle for eight years.
The Widnes forward admitted he was shocked to hear of Mullen’s positive test.
“It did [shock me] definitely. I just want to leave his business up to him but he’s a champion guy and it seems like he’s going to get a decent suspension,” Houston said.
“It’s such a shame. He’s been such a good guy for that club for some time. I know his family really well, it’s hard to take.
“I don’t know the full details really so I can’t go into it but he’s one of the good guys of the game and it’s such a shame to see he won’t be playing.
“It’s hard when you’re in that halfback position you’re getting plenty of traffic coming your way and always trying to put weight on and be strong because you’ve got to deal with forwards running at you all the time.
“To be fair he’s done it well for part of his career to play above his weight. It’s just a shame injuries force people to do something, whether he did or not.”