Arbeloa, in his first managerial role at senior level, took charge of Real Madrid after Xabi Alonso, another former Mourinho player, was sacked two weeks ago following a brief tenure.
Mourinho, who coached Real Madrid from 2010 to 2013, spoke warmly of his former defender, referring to him as one of his "favourites" on a personal level.
"Arbeloa is my boy; he's not just a former player of mine," Mourinho said at a press conference on Tuesday. "He is one of my favourite players on a personal level. He wasn't the best footballer I had at Madrid, but he was one of the best men. He's the last person I would put pressure on."
When asked if inexperience could be a factor against Arbeloa, who rose from coaching Real's youth academy to the senior squad, Mourinho dismissed the notion, wishing his former charge success at Real Madrid - but not on Wednesday.
"Arbeloa can coach any club in the world, and I want him to do well at Real, except for tomorrow."
Real Madrid head into the fixture on the back of a commanding 6-1 victory against Monaco last week, which moved them to third in the standings with 15 points. They trail Bayern Munich by three and leaders Arsenal by six as they close in on a top-eight finish and direct progression to the round of 16.
In stark contrast, Benfica find themselves in serious trouble, sitting 29th and in the elimination zone on six points. Mourinho's side need to beat Real to have any hope of a top-24 finish and a spot in the knockout-phase playoffs.
Mourinho acknowledged the uphill task facing his side but maintained confidence in Benfica's approach.
"We need to score one more goal than our opponents to win - that's obvious," he said. "We have to play to score, but we can't suffer too much in the process. Balance will be key. Without balance, Madrid will punish us."
The Portuguese manager also shared his fondness for Alonso.
"Xabi is another one of my boys," Mourinho added. "I am thrilled when I see my former players achieve great things. What happened to Xabi at Madrid after I left doesn't concern me, but he has proved the level he can reach as a manager, and I'm certain he is happy now."
Mourinho, who led Real Madrid to the LaLiga title in 2011-12, as well as Copa del Rey and Spanish Super Cup triumphs, said Wednesday's game was a "kill or be killed" scenario for Benfica.
"We'll be playing against a great team and great players," he remarked. "We have nothing to lose, so we'll fight to stay in the competition by maintaining balance and playing our game."