Okay, but in my case the word
strong is an
adverb which describes
play.
So I'm still not sure, but I really thing
We must play more strongly should be the right form.
You are so right. I'm sorry.
This article is probably more appropriate, then: Adjectives Versus Adverbs: Three Degrees of Separation — Infoplease.com
Since 'strongly' is an adverb describing play, the proper form would be 'play more strongly.'
However, I agree with other/previous posts, this doesn't sound natural to me.
I think I'd choose one of these:
"We need to play better."
"We need to play more aggressively."
"We need to play with more speed and agility."
Of these, 'more aggressively' probably comes closest to the meaning of your orignial post.
Of course, if it were my son-in-law, he'd probably say, "C'mon guys, let's get these suckers - let's pound them ... are you with me???"