I think activating the £20 million buyback for Alvaro would make sense because A) Hes doing well statistically in his gametime for Benfica and B) He's 186 cms, 1 cm taller than Casemiro and 1 cm shorter than Lindelof... At first as I feel Amass is going to be better I wasnt sure, but with his height and improved defending stats from Benfica he can play leftback and if Amass did outdo him when he got to play, he could still cover and compete for LCB with Lisandro and the right footers, or as part of a back 3 if we were to use it.
I dont think there are many standout cheaper options that could play leftback and cover CB for a long period. I like Gasriowski at Valencia but he doesnt play every week and hes kind of the opposite, a CB who would be makeshift and have to learn to play LB