After 51 games he has just 2 HRs. He had 24 in 153 games last year and 24 in 161 games in 2016.
His isolated power is just .097. Last year it was .201 and in 2016 it was .193.
He has only 4 SBs while he has had at least 30 for 6 straight years. His OBP at .357 is below each of the last 2 years (.396 and .410). But, his 1Bs, BBs and HBPs as a % of PAs has not fallen that much. It is .289 this year while it was .296 and .308 the last two years. Also, it is .280 for his career, so he is still doing better on that than for his whole career. So he is on first base quite a bit, at least not anything out of the ordinary for him.
And we are not talking about a slight decrease in steals. If he plays 153 games this year like he did last year, he will only end up with 12 at his current rate. He had 32 last year.
If we triple 2.67, we get 8 for a full season (of 153 games, in comparison with last year). Baseball Reference shows the league average for 600 PAs. The league average is 11 and last year it was 12. So at the pace he is going, he will end up below average.
Also, he has played in every game. He has started 49 of the 51 games the Astros have played this year with 433 innings at 2B, which is about 8.8 per start. And his defensive WAR is 0.5. If we tripled that, we would get 1.5 and his highest for a full season has been 0.8. So if he is hurt, it is not keeping him out of the lineup and it is not preventing him from playing good defense.