Bro, this has happened to me in SoloQ several times. Especially at the start of the season, when ex-Diamond players don't have their ranks yet and they can be practically put against any rank. My first game in this situation was against a Diamond 5 Kata mid (I was B2 then). The result was unexpected. No matter how good she played, I ended up going 13/1 and stomping the **** out of her.
The reason is very simple: if you have a normal MMR for, say, Silver/Gold, the matchmaking system will try to pair you against players with similar MMR. If a Diamond player has their MMR around 1300, then, well, that's a boosting case. So, in my experience, never be discouraged. Seeing a high-elo guy and thinking 'OK GG this is over' has a great psychological impact.
Also, even if they are truly high-elo, there's a VEEERY small chance that you'll be able to outplay them.
I once played a 1vs1 against a Master. Same champion. We had equal skills on that champion (Anivia) and we kept killing each other EXACTLY at the same time and doing exactly the same combos. Why? Because I am an ex-Anivia OTP and he was playing her for the 4th time that season. Remember: experience on a specific champ can counter even Diamonds XD. Rarely, but it can.