I guess the 'issue' with both De Jong and Tielemans is that they are more replacements for Thiago (as said by others) and we'd probably be better off with someone who can provide some genuine athleticism to our midfield or at least be able to fill in for Fabinho.
That said, if Tielemans is really available for ~20M though, it feels like kind of a no brainer... unless he wants mad money and all sorts of guarantees. He'd probably jump at the chance to ditch Leicester for us though.