To put it simply...
If Joshua comes in with overly heavy muscles and isn t light on his feet, he s done.
To answer this question in more detail...
Ruiz is an attacking counter-puncher. Historically, the fighters that beat this sub style are the pure mobile boxer...or the mobile boxer-puncher. Another recent example of a counter-puncher is Juan Manuel Marquez - neutral counter-puncher. Marquez was always trouble versus fighters that stayed in front of him or attacked him (Pacquiao, Juan Diaz, Rocky Juarez), but Marquez struggled with boxers that moved laterally after punching and / or escaped to the outside (Floyd Mayweather, Timothy Bradley...and even struggling against someone like Freddie Norwood).
Joshua has to use 2 / 3 types of "jabs" against Ruiz:
(a hard jab to head / body when he sees Ruiz slightly out of position, a soft / touch jab when he s moving his feet and circling away, and possibly a pawing-stiff arm intended to stop Ruiz s slips + counter rushes).
Sometimes, he has to follow the hard jab with a hard 2...or he has to throw the 2 without a jab...but ALWAYS moving laterally and never standing in front of Ruiz. And he HAS to move his head along with his feet once he throws his combinations.
Boxing from the outside, clinching Ruiz on the inside, and mixing in occasional 1-2-3 punch combos going forward (to change up his pattern) is what he s going to have to do for about 6-7 rounds. If he s able to do this, Ruiz would be gassed enough in later rounds not to have that punching explosion.
But if Joshua isn t able to do this for more than 6 rounds, the fight may turn into a fire-fight in the latter stages.
I can t call this fight. Ruiz is probably coming in lighter himself (with slightly faster feet). I think it s a 50-50 fight, and depends on the style that Joshua uses. If he boxes for most of the fight, he wins. If he tries to go toe-to-toe, he ll get caught again.