He wasn't missing, he still played every game he was fit, and probably ran more than any other player in the team during those games. You might not think that matters but it really does, as it contributes to pressure, and putting pressure on the opponents gets you the ball back quicker. It also freed up Crooks to work with Jones etc, if he wasn't as good or as fit/ doing a covering job then their stats would have suffered.
His average rating was second behind Crooks, his assists second behind Jones, 4th top scorer (ahead of a few strikers), joint second MOM, pass percentage was 4th of those who played more than 1000 minutes. A lot less yellows than Crooks, Jones and Howson, less bans etc.
Then consider what all those stats would have been if he played every game last year under Wilder, and if he had a decent player on the left beside him (he has both this year).
He's not going to be Iniesta.
If Spence is worth 12.5m to us + 7.5m of addons we might not get then Tav has to be worth at least 20m up front. We'd have to put £10m towards a less effective midfielder, but it would put £10m in the striker pot, which is getting more important by the day.
There's no point selling for 10-15m, we would get that or more next year, plus retain a good player on fair wages for a year (which is worth a lot).