Did Bayern's loss to Borussia Dortmund set off alarm bells in Munich?
Their resolve may just have been strengthened at the weekend, after their 3-0 loss at home to Borussia Dortmund.
Kroos was left on the bench for that one, and his replacement Bastian Schweinsteiger had a real game to forget.
Schweinsteiger has missed most of the campaign through injury, and his return was seen as a potential reason Bayern could cope without Kroos - but the preference would surely be to keep the latter.
Not only is Kroos less injury riddled and younger than Schweinsteiger, but he seems to get the philosophy of manager Pep Guardiola.
Schweinsteiger on the other hand has struggled to adjust under Guardiola. When there were grumblings about the Spanish boss early in the season, the German international was one who said he simply didn't know what his plans were for him, as he was shunted from position to position.
He has bought into Guardiola a lot more since then, hailing his achievement as 'incredible', but his performance against Dortmund was an underwhelming one.
The midfielder lasted the entire game, a match in which he made just one tackle and no interceptions, nor did he create any goalscoring opportunities, while the worst part of Schweinsteiger's day was that he was dispossessed four times.
Given the high profile nature of the defeat, and that Schweinsteiger had a week of rest ahead of the game due to his suspension against United, its logical Guardiola and Bayern will reflect on whether he has a long-term future at the German champions.
Kroos, by contrast, looked sharp when he came on, completing 94 per cent of his passes in his 21 minutes on the pitch compared to Schweinsteiger's 84 per cent. He was not dispossessed at all, and went onto create two chances for Bayern who at the stage he came on were already 3-0 down.
His bright cameo, contrasted with Schweinsteiger's unimpressive performance, may have helped convince Bayern that Kross is more important to them than ever, and lead them to pull out all the stops to persuade him to sign a new deal.