The former Valencia coach is expected to be formally announced as McClaren's assistant at St James' Park later this week, but has already spoken about his reasons for accepting a new challenge on Tyneside.
Cathro told BBC Sport: "It's a forward step to a different type of football. Not stronger or weaker, different.
"I wanted a fresh challenge and didn't want to just sit it out over there and wait for the right manager's job to come up.
"To come into British football as a manager with no experience of working with English-speaking players is perhaps not advisable anyway, so this makes perfect sense.
"Steve coming in has brought a freshness and enthusiasm to get the club back to where it should be and serve the love that exists for it.
"I'm convinced that it's a club willing to grow and would be comfortable higher in the league. It's now ready to do that.
"I'm going to watch Valencia in the Champions League and in La Liga and enjoy them playing in wonderful games in great stadiums.
"But I've no fear of missing out because the intention is to go to that level again. If that was with Newcastle, that would be perfect, but if not, I'll certainly be back working abroad in the next five to 10 years."