Looking at the Flight Schedules today, I'm not seeing any 48 min connections in SEA for EUG-SEA-SNA. I show a 1:10 departure from EUG arrive in SNA at 5:53 PM, with a 1 hr 1 min layover in SEA. Then I show a 5:21 PM departure from EUG, arriving in SNA at 9:59 PM, with a 43 min layover in SEA.
If you arrive on time, 43 min is enough time. The earlier options shows EUG-SEA arriving at A2 and SEA-SNA departing from A1. If that holds true, you could even make a 15 min connection. The later option shows EUG-SEA arriving at A2 and SEA-SNA departing from A3. But it's also too early for this gate info to be reliable. If you have to go between the S Gates or a far end B gate to a high number A gate, then 43 minutes could be a little tight if you're not a fast walker but it's still "doable."