The teenage winger's move was confirmed on Monday.
Tavares was a substitute in a first-team pre-season friendly against Dutch club Volendam last summer when only 15, but he has not appeared in a competitive fixture at senior level.
"Whilst we are incredibly sad to be losing a young academy player of Ajay's talent, our overriding sense is one of immense pride," said Knapper.
"Ajay's progress has been a testament to the fantastic work of everyone connected with our academy, who have impacted his journey and contributed to this wonderful achievement... and we couldn't be prouder to see him take this step to one of the most iconic football clubs and academies in world football."
Knapper added, "On an international level, he has represented England at under-15, under-16 and under-17 levels, and we've no doubt he'll continue to make great steps for both club and country in the years to come."