Taylor's DPAR was 8.6, his DVOA was .9% and he caught 65% of the passes thrown his way. The Vikings would be lucky to get that out of Johnson, considering his statistics have declined the past three years and he's not going to get any younger. If Johnson is content to be a cheap veteran presence for a year while imparting his wisdom to Troy Williamson, Sidney Rice and Aundrae Allison, that's one thing. If he expects to be a centerpiece of an offensive attack, get a decent size contract and generally be treated like a star, the Vikings should let him go elsewhere, since he no longer plays like a star.
[Explanations of DPAR can be found here, while the DVOA explanation is here.]