Wales international Ramsey is still sidelined with the hamstring injury sustained in the quarter-final second-leg win over Napoli and although he could return to action before moving to Juventus at the end of the season, he remains unfit.
Monreal, meanwhile, has a calf problem and will face a late fitness test ahead of the semi-final first leg at Emirates Stadium.
Denis Suarez is out with a groin injury, while Hector Bellerin (knee), Rob Holding (knee) and Danny Welbeck (ankle) are still recovering from long-term problems.
Unai Emery's men knocked Napoli out in the quarterfinals 3-0 on aggregate, but they've suffered a dip in form domestically since that triumph.
Three consecutive Premier League defeats has left Arsenal on the verge of missing out on a place in the top four and thusly a spot in next season's Champions League.
However, winning the Europa League is an alternative way for the Gunners to gain entry to the competition, with a final berth against either Eintracht Frankfurt or Chelsea to look forward to if they can get past Valencia.
The Spanish outfit also head into the encounter off the back of successive losses in La Liga and Emery will be expected to oversee a comfortable home win this evening.
Arsenal will then turn their attention back to the Premier League, with Brighton set to arrive at the Emirates on Sunday.