Pace isn't the be all and end all. It's endurance, aggression, application and strength as well. The latter can be worked on more than raw pace. Plus ball control, delivery, etc. Plenty of over examples besides Alonso, there's just not many PL sides to compare to. Beckham is one of the best wingers the league has ever seen and wasn't quick at all. Many successful wing backs in Serie A who weren't overly quick. Park was a wingback before we signed him and he wasn't quick, just a very willing and aggressive runner. Evra was successful at Juve well into his 30s when his pace had diminished. Young as well at Inter. Milner, although more at FB.
Yes you would probably want pacey alternatives and at LW or RWB for the balance of the side. We barely have any suitable players for the wing back or wide forward roles and will likely need a fair few over the next few windows if we stick with Amorim. Alvaro would be a great addition, in combination with some other options. Definitely has the ball playing ability to potential slot in at other positions as well. And he has the frame to develop physically.
If we have a buyback option (I can't remember if we do) then it's a no-brainer imo.