Correct, the enthusiasm always tends to get back to 5 both from higher and from lower values, but I would bet that it will definitely take longer than that.
Last year my team had the enthusiasm at 4 at one point, and it took ages to climb back to 5 playing with normal attitude... I'd say at least four games. And, what's worst, if southpaw's enthusiasm is 8 now, it will surely be a very low 4 after a CT game, or possibly a very high 3..... very far from 5, anyway.